Reduced diameter optical fiber with improved microbending

ABSTRACT

An optical fiber is provided that includes a core region and a cladding region. The core region is formed of silica glass doped with chlorine and/or an alkali metal. The cladding region surrounds the core region and includes an inner cladding directly adjacent to the core region, an outer cladding surrounding the inner cladding, and a trench region disposed between the inner cladding and the outer cladding in a radial direction. The trench region has a volume of about 30% Δ-micron 2  or greater. Additionally, the optical fiber has an effective area at 1550 nm of about 100 micron 2  or less.

This Application is a divisional of U.S. patent application Ser. No.17/184,909 filed on Feb. 25, 2021, which claims the benefit of priorityto Dutch Patent Application No. 2025269 filed on Apr. 3, 2020, whichclaims priority from U.S. Provisional Patent Application Ser. No.62/991,231 filed on Mar.18, 2020, the contents of which are relied uponand incorporated herein by reference in their entirety.

FIELD OF THE DISCLOSURE

This disclosure pertains to optical fibers. More particularly, thisdisclosure pertains to optical fiber cables configured for submarineenvironments. Most particularly, this disclosure pertains to opticalfibers having reduced diameters without a significant increase inmicrobending sensitivity.

BACKGROUND OF THE DISCLOSURE

Submarine cables are designed to carry telecommunication signals acrossstretches of land ocean and sea. Over the past several years, there hasbeen a dramatic increase in telecommunications signals over submarinecables, with greater than ninety percent of inter-continentalcommunication signals currently being transmitted over these cables.Thus, the demand for the transmission capacity of such submarine cableshas increased, driven by the growth of internet traffic among differentcontinents.

Traditional approaches to increase the transmission capacity ofsubmarine cables include wavelength division multiplexing, to increasethe number of transmission channels, and advanced modulation formats, toincrease the data rate per channel. However, the number of channels andchannel data rate are nearly at the practical limits, thus renderingthese approaches no longer practical. Another possible approach toincrease the transmission capacity of submarine cables is to increasethe number of fibers within the cables by increasing the overalldiameter of the cables. However, this approach is also not practical asthe diameter of submarine cables is limited in order to provide easydeployment of the cables. Increasing the diameter of the submarine cablemakes the cables harder to manage due to the increased weight and thelimited storage capacity on the ships that deploy undersea opticalcables.

SUMMARY

The present disclosure provides optical fibers having reduced diametersto increase the fiber count in submarine cables, thus allowing thediameter of the submarine cables to remain at an acceptable size foreasy deployment. In particular, the optical fibers disclosed herein havea reduced glass diameter and/or a reduced coating thickness while stillmaintaining microbending characteristics needed for long-haultransmission. More specifically, the optical fibers disclosed herewithprovide low attenuation, low microbending sensitivity, and high punctureresistance in a compact form. The reduced glass diameter and/or reducedcoating thickness may be used to increase the fiber density withinstandard submarine cable designs. The microbending properties of suchreduced diameter optical fibers, as disclosed herein, are achieved byco-optimizing the coating properties of the fibers with the dimensionsof a depressed cladding layer in the refractive index profile to inhibitthe leakage of the optical signal. The higher modulus of the secondarycoating improves the puncture resistance and handleability of the fiber,despite the smaller cross-sectional area.

The present description extends to an optical fiber having a core regioncomprising silica glass doped with chlorine and/or an alkali metal. Theoptical fiber further includes a cladding region surrounding the coreregion, the cladding region comprising an inner cladding directlyadjacent to the core region, an outer cladding surrounding the innercladding, and a trench region disposed between the inner cladding andthe outer cladding in a radial direction, the trench region having avolume of about 30% Δ-micron² or greater. Additionally, the opticalfiber has an effective area at 1550 nm of about 100 micron² or less.

The present description further extends to an optical fiber having acore region comprising silica glass doped with chlorine and/or an alkalimetal and a cladding region surrounding the core region. The claddingregion comprises an inner cladding directly adjacent to the core region,an outer cladding surrounding the inner cladding, and a trench regiondisposed between the inner cladding and the outer cladding in a radialdirection, the trench region having a volume of about 30% Δ-micron² orgreater. The optical fiber further includes a primary coatingsurrounding the cladding region and a secondary coating surrounding theprimary coating. The primary coating having an in situ modulus of about0.5 MPa or less, and the secondary coating having an in situ modulus ofabout 1500 MPa or more. A diameter of the secondary coating is about 210microns or less.

The present description further extends to a multicore optical fiberhaving a first core comprising silica glass doped with chlorine and/oran alkali metal, a first inner cladding surrounding the first core, anda first outer cladding surrounding the first inner cladding andcomprising a first trench region having a volume of about 30% Δ-micron²or greater. Additionally, the multicore optical fiber has a second corecomprising silica glass doped with chlorine and/or an alkali metal, asecond inner cladding surrounding the second core, and a second outercladding surrounding the second inner cladding and comprising a secondtrench region having a volume of about 30% Δ-micro² or greater. A commoncladding surrounds the first core and the second core. Furthermore, thefirst core and the second core each have an effective area at 1550 nm ofabout 100 micron² or less.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary and are intendedto provide an overview or framework to understand the nature andcharacter of the claims.

The accompanying drawings are included to provide a furtherunderstanding and are incorporated in and constitute a part of thisspecification. The drawings are illustrative of selected aspects of thepresent disclosure, and together with the description serve to explainprinciples and operation of methods, products, and compositions embracedby the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a coated optical fiber according toembodiments of the present disclosure;

FIG. 2 is a schematic view of an optical fiber ribbon according toembodiments of the present disclosure;

FIG. 3 is a schematic view of an optical fiber cable according toembodiments of the present disclosure;

FIG. 4 is a schematic view of a cross-section of an optical fiberaccording to embodiments of the present disclosure;

FIG. 5 depicts a relative refractive index profile of an optical fiberaccording to embodiments of the present disclosure;

FIGS. 6A-6E depict relative refractive index profiles of optical fibersaccording to embodiments of the present disclosure;

FIG. 7 depicts a plot of radiative loss vs. cladding radius for twooptical fibers;

FIG. 8A depicts a schematic view of a multicore optical fiber accordingto embodiments of the present disclosure;

FIG. 8B depicts schematic views of multicore optical fibers according toembodiments of the present disclosure;

FIG. 8C depicts a schematic view of a multicore optical fiber accordingto embodiments of the present disclosure;

FIG. 9 depicts a schematic view of a multicore optical fiber accordingto embodiments of the present disclosure;

FIG. 10 depicts a plot of crosstalk vs. core spacing for three multicoreoptical fibers; and

FIG. 11 depicts relative refractive index profiles of multicore opticalfiber cores according to embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure is provided as an enabling teaching and can beunderstood more readily by reference to the following description,drawings, examples, and claims. To this end, those skilled in therelevant art will recognize and appreciate that many changes can be madeto the various aspects of the embodiments described herein, while stillobtaining the beneficial results. It will also be apparent that some ofthe desired benefits of the present embodiments can be obtained byselecting some of the features without utilizing other features.Accordingly, those who work in the art will recognize that manymodifications and adaptations are possible and can even be desirable incertain circumstances and are a part of the present disclosure.Therefore, it is to be understood that this disclosure is not limited tothe specific compositions, articles, devices, and methods disclosedunless otherwise specified. It is also to be understood that theterminology used herein is for the purposes of describing particularaspects only and is not intended to be limiting.

In this specification and in the claims which follow, reference will bemade to a number of terms which shall be defined to have the followingmeanings:

“Optical fiber” refers to a waveguide having a glass portion surroundedby a coating. The glass portion includes a core and a cladding and isreferred to herein as a “glass fiber”.

“Radial position”, “radius”, or the radial coordinate “r” refers toradial position relative to the centerline (r=0) of the fiber.

“Refractive index” refers to the refractive index at a wavelength of1550 nm, unless otherwise specified.

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and radius. For relative refractiveindex profiles depicted herein as having step boundaries betweenadjacent core and/or cladding regions, normal variations in processingconditions may preclude obtaining sharp step boundaries at the interfaceof adjacent regions. It is to be understood that although boundaries ofrefractive index profiles may be depicted herein as step changes inrefractive index, the boundaries in practice may be rounded or otherwisedeviate from perfect step function characteristics. It is furtherunderstood that the value of the relative refractive index may vary withradial position within the core region and/or any of the claddingregions. When relative refractive index varies with radial position in aparticular region of the fiber (e.g. core region and/or any of thecladding regions), it is expressed in terms of its actual or approximatefunctional dependence, or its value at a particular position within theregion, or in terms of an average value applicable to the region as awhole. Unless otherwise specified, if the relative refractive index of aregion (e.g. core region and/or any of the cladding regions) isexpressed as a single value or as a parameter (e.g. Δ or Δ %) applicableto the region as a whole, it is understood that the relative refractiveindex in the region is constant, or approximately constant, andcorresponds to the single value, or that the single value or parameterrepresents an average value of a non-constant relative refractive indexdependence with radial position in the region. For example, if “i” is aregion of the glass fiber, the parameter Δ_(i) refers to the averagevalue of relative refractive index in the region as defined by Eq. (1)below, unless otherwise specified. Whether by design or a consequence ofnormal manufacturing variability, the dependence of relative refractiveindex on radial position may be sloped, curved, or otherwisenon-constant.

“Relative refractive index,” as used herein, is defined in Eq. (1) as:

$\begin{matrix}{{{\Delta_{i}\left( r_{i} \right)}\%} = {100\frac{\left( {n_{i}^{2} - n_{ref}^{2}} \right)}{2n_{i}^{2}}}} & (1)\end{matrix}$

where n_(i) is the refractive index at radial position r_(i) in theglass fiber, unless otherwise specified, and n_(ref) is the refractiveindex of pure silica glass, unless otherwise specified. Accordingly, asused herein, the relative refractive index percent is relative to puresilica glass, which has a value of 1.444 at a wavelength of 1550 nm. Asused herein, the relative refractive index is represented by Δ (or“delta”) or Δ % (or “delta %) and its values are given in units of “%”,unless otherwise specified. Relative refractive index may also beexpressed as Δ(r) or Δ(r) %.

The average relative refractive index (Δ_(ave)) of a region of the fiberis determined from Eq. (2):

$\begin{matrix}{\Delta_{ave} = {\int_{r_{inner}}^{r_{outer}}\frac{{\Delta(r)}{dr}}{\left( {r_{outer} - r_{inner}} \right)}}} & (2)\end{matrix}$

where r_(inner) is the inner radius of the region, r_(router) is theouter radius of the region, and Δ(r) is the relative refractive index ofthe region.

The refractive index of an optical fiber profile may be measured usingcommercially available devices, such as the IFA-100 Fiber Index Profiler(Interfiber Analysis LLC, Sharon, MA USA) or the S14 Refractive IndexProfiler (Photon Kinetics, Inc., Beaverton, OR USA). These devicesmeasure the refractive index relative to a measurement reference index,n(r)−n_(meas), where the measurement reference index nmeas is typicallya calibrated index matching oil or pure silica glass. The measurementwavelength may be 632.5 nm, 654 nm, 677.2 nm, 654 nm, 702.3 nm, 729.6nm, 759.2 nm, 791.3 nm, 826.3 nm, 864.1 nm, 905.2 nm, 949.6 nm, 997.7nm, 1050 nm, or any wavelength therebetween. The absolute refractiveindex n(r) is then used to calculate the relative refractive index asdefined by Eq. (1).

The term “α-profile” or “alpha profile” refers to a relative refractiveindex profile Δ(r) that has the functional form defined in Eq. (3):

$\begin{matrix}{{\Delta(r)} = {{\Delta\left( r_{0} \right)}\left\lbrack {1 - \left\lbrack \frac{❘{r - r_{0}}❘}{\left( {r_{z} - r_{0}} \right)} \right\rbrack^{\alpha}} \right\rbrack}} & (3)\end{matrix}$

where r_(o) is the radial position at which Δ(r) is maximum, Δ(r₀)>0,r_(z)>r₀ is the radial position at which Δ(r) decreases to its minimumvalue, and r is in the range r_(i)≤r≤r_(f), where f_(i) is the initialradial position of the α-profile, r_(f) is the final radial position ofthe α-profile, and α is a real number. Δ(r₀) for an α-profile may bereferred to herein as Δ_(max) or, when referring to a specific region iof the fiber, as Δ_(imax). When the relative refractive index profile ofthe fiber core region is described by an α-profile with r₀ occurring atthe centerline (r=0), r_(z) corresponding to the outer radius r₁ of thecore region, and Δ₁(r₁)=0, Eq. (3) simplifies to Eq. (4):

$\begin{matrix}{{\Delta_{1}(r)} = {\Delta_{1\max}\left\lbrack {1 - \left\lbrack \frac{r}{r_{1}} \right\rbrack^{\alpha}} \right\rbrack}} & (4)\end{matrix}$

When the core region has an index described by Eq. (4), the outer radiusr₁ can be determined from the measured relative refractive index profileby the following procedure. Estimated values of the maximum relativerefractive index Δ_(1max), α, and outer radius r_(lest) are obtainedfrom inspection of the measured relative refractive index profile andused to create a trial function Δ_(trial) between r=−r_(1est) andr=r_(1est). Relative refractive index profiles of representative glassfibers having cores described by an α-profile, in accordance withembodiments of the present disclosure, are shown in FIGS. 5 and 6 .

“Trench volume” is defined as:

$\begin{matrix}{V_{Trench} = {❘{2{\int_{r_{{Trench},{inner}}}^{r_{{Trench},{outer}}}{{\Delta_{Trench}(r)}{rdr}}}}❘}} & (5)\end{matrix}$

where r_(Trench,inner) is the inner radius of the trench region of therefractive index profile, r_(Trench,outer) is the outer radius of thetrench region of the refractive index profile, Δ_(Trench)(r) is therelative refractive index of the trench region of the refractive indexprofile, and r is radial position in the fiber. Trench volume is inabsolute value and a positive quantity and will be expressed herein inunits of % Δmicron², % Δ-micron², % Δ-μm², or % Δμm², whereby theseunits can be used interchangeably herein. A trench region is alsoreferred to herein as a depressed-index cladding region and trenchvolume is also referred to herein as V₃.

The “mode field diameter” or “MFD” of an optical fiber is defined in Eq.(6) as:

$\begin{matrix}{{MFD} = {2w}} & (6)\end{matrix}$$w^{2} = {2\frac{\int_{0}^{\infty}{\left( {f(r)} \right)^{2}{rdr}}}{\int_{0}^{\infty}{\left( \frac{{df}(r)}{dr} \right)^{2}{rdr}}}}$

where f(r) is the transverse component of the electric fielddistribution of the guided optical signal and r is radial position inthe fiber. “Mode field diameter” or “MFD” depends on the wavelength ofthe optical signal and is reported herein for wavelengths of 1310 nm,1550 nm, and 1625 nm. Specific indication of the wavelength will be madewhen referring to mode field diameter herein. Unless otherwisespecified, mode field diameter refers to the LP₀₁ mode at the specifiedwavelength. “Effective area” of an optical fiber is defined in Eq. (7)as:

$\begin{matrix}{A_{eff} = \frac{2{\pi\left\lbrack {\int_{0}^{\infty}{\left( {f(r)} \right)^{2}{rdr}}} \right\rbrack}^{2}}{\int_{0}^{\infty}{\left( {f(r)} \right)^{4}{rdr}}}} & (7)\end{matrix}$

where f(r) is the transverse component of the electric field of theguided optical signal and r is radial position in the fiber. “Effectivearea” or “A_(eff)” depends on the wavelength of the optical signal andis understood herein to refer to a wavelength of 1550 nm.

The term “attenuation,” as used herein, is the loss of optical power asthe signal travels along the optical fiber. Attenuation was measured asspecified by the IEC-60793-1-40 standard, “Attenuation measurementmethods.”

The bend resistance of an optical fiber, expressed as “bend loss”herein, can be gauged by induced attenuation under prescribed testconditions as specified by the IEC-60793-1-47 standard, “Measurementmethods and test procedures—Macrobending loss.” For example, the testcondition can entail deploying or wrapping the fiber one or more turnsaround a mandrel of a prescribed diameter, e.g., by wrapping 1 turnaround either a 15 mm, 20 mm, or 30 mm or similar diameter mandrel (e.g.“1×15 mm diameter bend loss” or the “1×20 mm diameter bend loss” or the“1×30 mm diameter bend loss”) and measuring the increase in attenuationper turn.

“Cable cutoff wavelength,” or “cable cutoff,” as used herein, refers tothe 22 m cable cutoff test as specified by the IEC 60793-1-44 standard,“Measurement methods and test procedures-Cut-off wavelength.”

The optical fibers disclosed herein include a core region, a claddingregion surrounding the core region, and a coating surrounding thecladding region. The core region and cladding region are glass. Thecladding region includes multiple regions. The multiple cladding regionsare preferably concentric regions. The cladding region includes an innercladding region, a depressed-index cladding region, and an outercladding region. The inner cladding region surrounds and is directlyadjacent to the core region. The depressed-index cladding regionsurrounds and is directly adjacent to the inner cladding region suchthat the depressed-index cladding region is disposed between the innercladding and the outer cladding in a radial direction. The outercladding region surrounds and is directly adjacent to thedepressed-index cladding region. The depressed-index cladding region hasa lower relative refractive index than the inner cladding and the outercladding region. The depressed-index cladding region may also bereferred to herein as a trench or trench region. The relative refractiveindex of the inner cladding region may be less than, equal to, orgreater than the relative refractive index of the outer cladding region.The depressed-index cladding region may contribute to a reduction inbending losses and microbending sensitivity. The core region, innercladding region, depressed-index cladding region, and outer claddingregion are also referred to as core, cladding, inner cladding,depressed-index cladding, and outer cladding, respectively.

Whenever used herein, radial position r₁ and relative refractive indexΔ₁ or Δ₁(r) refer to the core region, radial position r₂ and relativerefractive index Δ₂ or Δ₂ (r) refer to the inner cladding region, radialposition r₃ and relative refractive index Δ₃ or Δ₃(r) refer to thedepressed-index cladding region, radial position r₄ and relativerefractive index Δ₄ or Δ₄(r) refer to the outer cladding region, radialposition r₅ refers to the primary coating, radial position r₆ refers tothe secondary coating, and the radial position r₇ refers to the optionaltertiary coating.

The relative refractive index Δ₁(r) has a maximum value Δ_(1max) and aminimum value Δ_(1min). The relative refractive index Δ₂(r) has amaximum value Δ_(2max) and a minimum value Δ_(2min). The relativerefractive index Δ₃(r) has a maximum value Δ_(3max) and a minimum valueΔ_(3min). The relative refractive index Δ₄(r) has a maximum valueΔ_(4max) and a minimum value Δ_(4min). In embodiments in which therelative refractive index is constant or approximately constant over aregion, the maximum and minimum values of the relative refractive indexare equal or approximately equal. Unless otherwise specified, if asingle value is reported for the relative refractive index of a region,the single value corresponds to an average value for the region.

It is understood that the central core region is substantiallycylindrical in shape and that the surrounding inner cladding region,depressed-index cladding region, outer cladding region, primary coating,and secondary coating are substantially annular in shape. Annularregions are characterized in terms of an inner radius and an outerradius. Radial positions r₁, r₂, r₃, r₄, r₅, r₆ and r₇ refer herein tothe outermost radii of the core, inner cladding, depressed-indexcladding, outer cladding, primary coating, secondary coating, andtertiary coating, respectively. The radius r₆ also corresponds to theouter radius of the optical fiber in embodiments without a tertiarycoating. When a tertiary coating is present, the radius r₇ correspondsto the outer radius of the optical fiber.

When two regions are directly adjacent to each other, the outer radiusof the inner of the two regions coincides with the inner radius of theouter of the two regions. The optical fiber, for example, includes adepressed-index cladding region surrounded by and directly adjacent toan outer cladding region. The radius r₃ corresponds to the outer radiusof the depressed-index cladding region and the inner radius of the outercladding region. The relative refractive index profile also includes adepressed-index cladding region surrounding and directly adjacent to aninner cladding region. The radial position r₂ corresponds to the outerradius of the inner cladding region and the inner radius of thedepressed-index cladding region. Similarly, the radial position r₁corresponds to the outer radius of the core region and the inner radiusof the inner cladding region.

The difference between radial position r₂ and radial position r₁ isreferred to herein as the thickness of the inner cladding region. Thedifference between radial position r₃ and radial position r₂ is referredto herein as the thickness of the depressed-index cladding region. Thedifference between radial position r₄ and radial position r₃ is referredto herein as the thickness of the outer cladding region. The differencebetween radial position r₅ and radial position r₄ is referred to hereinas the thickness of the primary coating. The difference between radialposition r₆ and radial position r₅ is referred to herein as thethickness of the secondary coating.

As will be described further hereinbelow, the relative refractiveindices of the core region, inner cladding region, depressed-indexcladding region, and outer cladding region may differ. Each of theregions may be formed from doped or undoped silica glass. Variations inrefractive index relative to undoped silica glass are accomplished byincorporating updopants or downdopants at levels designed to provide atargeted refractive index or refractive index profile using techniquesknown to those of skill in the art. Updopants are dopants that increasethe refractive index of the glass relative to the undoped glasscomposition. Downdopants are dopants that decrease the refractive indexof the glass relative to the undoped glass composition. In oneembodiment, the undoped glass is silica glass. When the undoped glass issilica glass, updopants include Cl, Br, Ge, Al, P, Ti, Zr, Nb, and Ta,and downdopants include F and B. Regions of constant refractive indexmay be formed by not doping or by doping at a uniform concentration overthe thickness of the region. Regions of variable refractive index areformed through non-uniform spatial distributions of dopants over thethickness of a region and/or through incorporation of different dopantsin different regions. Downdoping can also be accomplished byincorporating voids in silica glass. Voids correspond to localizedregions filled with air or other gas (e.g. N₂, Ar, SO₂, CO₂, Kr, O₂)and/or evacuated spaces that extend for a length less than the fulllength of the glass fiber. The voids are preferably distributed randomlyor non-periodically along the length of the glass fiber.

Values of Young's modulus, % elongation, and tear strength refer tovalues as determined under the measurement conditions by the proceduresdescribed herein.

Reference will now be made in detail to illustrative embodiments of thepresent description.

One embodiment relates to an optical fiber. The optical fiber includes aglass fiber surrounded by a coating. An example of an optical fiber isshown in schematic cross-sectional view in FIG. 1 . Optical fiber 10includes glass fiber 11 surrounded by primary coating 16 and secondarycoating 18. In some embodiments, secondary coating 18 may include apigment. Further description of glass fiber 11, primary coating 16, andsecondary coating 18 is provided below. Additionally, one or moretertiary ink layers may surround secondary coating 18.

FIG. 2 illustrates an optical fiber ribbon 30, which may include aplurality of optical fibers 20 and a matrix 32 encapsulating theplurality of optical fibers. Optical fibers 20 each include a coreregion, a cladding region, a primary coating, and a secondary coating asdescribed above. Optical fibers 20 may also include a tertiary coatingas noted above.

As shown in FIG. 2 , optical fibers 20 are aligned relative to oneanother in a substantially planar and parallel relationship. The opticalfibers in fiber optic ribbon 30 are encapsulated by the ribbon matrix 32in any of several known configurations (e.g., edge-bonded ribbon,thin-encapsulated ribbon, thick-encapsulated ribbon, or multi-layerribbon) by conventional methods of making fiber optic ribbons. Fiberoptic ribbon 30 in the embodiment of FIG. 2 contains twelve (12) opticalfibers 20. However, it is contemplated that any number of optical fibers20 (e.g., two or more) may be employed to form fiber optic ribbon 30 fora particular use. Ribbon matrix 32 has tensile properties similar to thetensile properties of a secondary coating and can be formed from thesame, similar, or different composition used to prepare a secondarycoating.

FIG. 3 illustrates an optical fiber cable 40 that includes a pluralityof optical fibers 20 surrounded by jacket 42. In some embodiments,optical fiber cable 40 is a submarine cable. Optical fibers 20 may bedensely or loosely packed into a conduit enclosed by an inner surface 44of jacket 42. The number of fibers placed in jacket 42 is referred to asthe “fiber count” of optical fiber cable 40. As discussed further below,the optical fibers of the present disclosure have a reduced diameter,thus providing a high “fiber count.”

The jacket 42 is formed from an extruded polymer material and mayinclude multiple concentric layers of polymers or other materials.Optical fiber cable 40 may include one or more strengthening members(not shown) embedded within jacket 42 or placed within the conduitdefined by inner surface 44. Strengthening members include fibers orrods that are more rigid than jacket 42. The strengthening member may bemade from metal, braided steel, glass-reinforced plastic, fiber glass,or other suitable material. Optical fiber cable 40 may include otherlayers surrounded by jacket 42 such as, for example, armor layers,moisture barrier layers, rip cords, etc. Furthermore, optical fibercable 40 may have a stranded, loose tube core or other fiber optic cableconstruction.

Glass Fiber

As shown in FIG. 1 , glass fiber 11 includes a core region 12 and acladding region 14, as is known in the art. Core region 12 has a higherrefractive index than cladding region 14, and glass fiber 11 functionsas a waveguide. In many applications, core region 12 and cladding region14 have a discernible core-cladding boundary. Alternatively, core region12 and cladding region 14 can lack a distinct boundary.

In some embodiments, core region 12 has a refractive index that varieswith distance from the center of the glass fiber. For example, coreregion 12 may have a relative refractive index profile with an α-profile(as defined by Eq. (3) above) with an α value that is greater than orequal to 2 and less than or equal to 100, or for example between 2 and10, between 2 and 6, between 2 and 4, between 4 and 20, between 6 and20, between 8 and 20, between 10 and 20, or between 10 and 40.

A schematic cross-sectional depiction of an exemplary optical fiber isshown in FIG. 4 . As discussed above, the optical fiber of FIG. 4 may beused in a submarine cable. In FIG. 4 , optical fiber 46 includes coreregion 48, cladding region 50, primary coating 56, and secondary coating58. Cladding region 50 includes inner cladding region 51,depressed-index cladding region 53, and outer cladding region 55. Atertiary layer (e.g. ink layer) optionally surrounds or is directlyadjacent to the secondary coating.

As discussed above, optical fiber 46 may have a reduced glass diameterand/or a reduced coating diameter. Such reduced diameter(s) may increasethe fiber density (e.g., “fiber count”) of optical fibers 46 when used,for example, in a standard submarine cable design. In order to providesuitable attenuation and microbending characteristics with the smallerdiameter profile of optical fiber 46, the properties of the fiber arespecifically tailored, as discussed further below.

A representative relative refractive index profile for a glass fiber,according to embodiments of the present disclosure, is shown in FIG. 5 .The profile of optical fiber 60 of FIG. 5 shows a core region (1) withouter radius r₁ and relative refractive index Ai with maximum relativerefractive index Δ_(1max), an inner cladding region (2) extending fromradial position r₁ to radial position r₂ and having relative refractiveindex Δ₂, a depressed-index cladding region (3) extending from radialposition r₂ to radial position r₃ and having relative refractive indexΔ₃, and an outer cladding region (4) extending from radial position r₃to radial position r₄ and having relative refractive index Δ₄. In theprofile of FIG. 5 , the depressed-index cladding region (3) may bereferred to herein as a trench and has a constant or average relativerefractive index that is less than the relative refractive indices ofthe inner cladding region (2) and the outer cladding region (4). Coreregion (1) has the highest average and maximum relative refractive indexin the profile. Core region (1) may include a lower index region at ornear the centerline (known in the art as a “centerline dip”) (notshown). Core region (1) may include a higher index region at or near thecenterline (referred to as a “centerline spike”) (not shown).

In the relative refractive index profile of FIG. 5 , the core region (1)of the glass fiber has an α-profile with an α value greater than orequal to 2 and less than or equal to 20. The radial position r₀(corresponding to Δ_(1max)) of the α-profile corresponds to thecenterline (r=0) of the fiber and the radial position r_(z) of theα-profile corresponds to the core radius r₁. In embodiments with acenterline dip, the radial position r₀ is slightly offset from thecenterline of the fiber. In some embodiments, the relative refractiveindex Δ₁ continuously decreases in the radial direction away from thecenterline. In other embodiments, relative refractive index Δ₁ variesover some radial positions between the centerline and r₁, and alsoincludes a constant or approximately constant value over other radialpositions between the centerline and r₁.

In FIG. 5 , transition region 61 from inner cladding region (2) todepressed-index cladding region (3) and transition region 62 fromdepressed-index cladding region (3) to outer cladding region (4) areshown as step changes. It is to be understood that a step change is anidealization and that transition region 61 and/or transition region 62may not be strictly vertical in practice as depicted in FIG. 5 .Instead, transition region 61 and/or transition region 62 may have aslope or curvature. When transition region 61 and/or transition region62 are non-vertical, the inner radius r₂ and outer radius r₃ ofdepressed-index cladding region (3) correspond to the mid-points oftransition regions 61 and 62, respectively. The mid-points correspond tohalf of the depth 63 of the depressed-index cladding region (3).

The relative ordering of relative refractive indices Δ₁, Δ₂, Δ₃, Δ₄ inthe relative refractive index profile shown in FIG. 5 satisfy theconditions Δ_(1max)>Δ₄>Δ₃ and Δ_(1max)>Δ₂>Δ₃. The values of Δ₂ and Δ₄may be equal or either may be greater than the other, but both Δ₂ and Δ₄are between Δ_(1max) and Δ₃.

The relative refractive indices Δ₁, Δ₂, Δ₃, and Δ₄ are based on thematerials used in the core region, inner cladding region,depressed-index cladding region, and outer cladding region. Adescription of these material with regard to the relative refractiveindices Δ₁, Δ₂, Δ₃, and Δ₄ is provided below.

Core Region

The core region comprises silica glass. The silica glass of the coreregion may be undoped silica glass, updoped silica glass, and/ordowndoped silica glass. Updoped silica glass includes silica glass dopedwith an alkali metal oxide (e.g. Na₂O, K₂O, Li₂O, Cs₂O, or Rb₂O) and/ora halogen. Downdoped silica glass includes silica glass doped withfluorine. In some embodiments, the silica glass of the core region isGe-free and/or Cl-free; that is the core region comprises silica glassthat lacks germanium and/or chlorine.

The core region may comprise silica glass doped with at least one alkalimetal, such as, lithium (Li), sodium (Na), potassium (K), rubidium (Rb),cesium (Cs) and/or francium (Fr). In some embodiments, the silica glassis doped with a combination of sodium, potassium, and rubidium. Thesilica glass may have a peak alkali concentration in the range fromabout 10 ppm to about 500, or in the range from about 20 ppm to about450 ppm, or in the range from about 50 ppm to about 300 ppm, or in therange from about 10 ppm to about 200 ppm, or in the range from about 10ppm to about 150 ppm. The alkali metal doping within the disclosedranges results in lowering of Rayleigh scattering, thereby proving alower optical fiber attenuation.

In some embodiments, the core region comprises silica glass doped withan alkali metal and doped with fluorine as a downdopant. FIGS. 5 and 6show exemplary embodiments of downdoped silica glass. The concentrationof fluorine in the glass fiber is in the range from about 0.1 wt % toabout 2.5 wt %, or in the range from about 0.25 wt % to about 2.25 wt %,or in the range from about 0.3 wt % to about 2.0 wt %.

In yet other embodiments, the core region comprises silica glass dopedwith a halogen such as chlorine. FIGS. 6B-6D show exemplary embodimentsof silica glass doped with chlorine. The concentration of chlorine inthe glass fiber is in a range from about 0.4 wt % to about 2.2 wt %, orabout 0.6 wt % to about 2.0 wt %. or about 1.0 wt % to about 1.9 wt %.or about 1.6 wt %, or about 1.8 wt %.

The radius r₁ of the core region is in the range from about from about3.0 microns to about 6.0 microns, or in the range from about 3.5 micronsto about 5.5 microns, or in the range from about 4.0 microns to about5.0 microns, or in the range from about 4.2 microns to about 4.7microns. In some embodiments, the core region includes a portion with aconstant or approximately constant relative refractive index that has awidth in the radial direction of at least 1.0 micron, or at least 2.0microns, or at least 3.0 microns, or at least 4.0 microns, or in therange from 1.0 microns to 4.0 microns, or in the range from 2.0 micronsto 3.0 microns. In some embodiments, the portion of the core regionhaving a constant or approximately constant relative refractive indexhas a relative refractive index of Δ_(1min).

The relative refractive index Δ₁ or Δ_(1max) of the core region is inthe range from about −0.15% to about 0.30%, or in the range from about−0.10% to about 0.20%, or in the range from about −0.05% to about 0.15%,or in the range from about 0% to about 0.10%. The minimum relativerefractive index Δ_(1min) of the core is in the range from about −0.20%to about −0.50%, or in the range from about −0.30% to about −0.40%, orin the range from about −0.32% to about 0.37%. The difference Δ_(1max)to Δ_(1min) is greater than 0.05%, or greater than 0.10%, or greaterthan 0.15%, or greater than 0.20%, or in the range from 0.05% to 0.40%,or in the range from 0.10% to 0.35%.

Inner Cladding Region

The inner cladding region is comprised of downdoped silica glass that isdoped with fluorine and/or silica glass with voids. The averageconcentration of downdopant in the inner cladding region is greater thanthe average concentration of downdopant in the core region. In someembodiments, the concentration of fluorine in the inner cladding regionis in the range from about 0.50 wt % to about 2.00 wt %, or in the rangefrom about 0.60 wt % to about 1.00 wt %, or in the range from about 0.70wt % to about 0.80 wt %.

The relative refractive index Δ₂ or Δ_(2max) of the inner claddingregion is in the range from about −0.20% to about −0.50%, or in therange from about −0.25% to about −0.45%, or in the range from about−0.30% to about −0.40%, or in the range from about −0.33% to about−0.37%. The relative refractive index Δ₂ is preferably constant orapproximately constant. The difference Δ_(1max)−Δ₂ (or the differenceΔ_(1max)−Δ_(2max)) is greater than about 0.25%, or greater than about0.30%, or greater than about 0.35%, or in the range from about 0.25% toabout 0.45%, or in the range from about 0.30% to about 0.40%.

The radius r₂ of the inner cladding region is in the range from about7.0 microns to about 15.0 microns, or in the range from about 7.5microns to about 13.0 microns, or in the range from about 8.0 microns toabout 12.0 microns, or in the range from about 8.5 microns to about 11.5microns, or in the range from about 9.0 microns to about 11.0 microns,or in the range from about 9.5 microns to about 10.5 microns. Thethickness r₂−r₁ of the inner cladding region is in the range from about3.0 microns to about 10.0 microns, or from about 4.0 microns to about9.0 microns, or from about 4.5 microns to about 7.0 microns.

Depressed-Index Cladding Region

The depressed-index cladding region comprises downdoped silica glass. Asdiscussed above, the preferred downdopant is fluorine. The concentrationof fluorine in the depressed- index cladding region is in the range fromabout 0.30 wt % to about 2.50 wt %, or in the range from about 0.60 wt %to about 2.25 wt %, or in the range from about 0.90 wt % to about 2.00wt %.

The relative refractive index Δ₃ or Δ_(3min) is in the range from about−0.30% to about −0.80%, or in the range from about −0.40% to about−0.70%, or in the range from about −0.50% to about −0.65%. The relativerefractive index Δ₃ is preferably constant or approximately constant.The difference Δ_(1max)−Δ₃ (or the difference AΔ_(1max)−Δ_(3min), or thedifference Δ₁−Δ₃, or the difference Δ₁−Δ_(3min)) is greater than about0.50%, or greater than about 0.55%, or greater than about 0.6%, or inthe range from about 0.50% to about 0.80%, or in the range from about0.55% to about 0.75%. The difference Δ₂−Δ₃ (or the differenceΔ₂−Δ_(3min), or the difference Δ_(2max)−Δ₃, or the differenceΔ_(2max)−Δ_(3min)) is greater than about 0.10%, or greater than about0.20%, or greater than about 0.30%, or in the range from about 0.10% toabout 0.60%, or in the range from about 0.20% to about 0.60%.

The inner radius of the depressed-index cladding region is r₂ and hasthe values specified above. The outer radius r₃ of the depressed-indexcladding region is in the range from about 10.0 microns to 20.0 microns,or in the range from about 12.0 microns to about 19.5 microns, or in therange from about 13.0 microns to about 19.0 microns, or in the rangefrom about 13.5 microns to about 18.5 microns, or in the range fromabout 14.0 microns to about 18.0 microns, or in the range from about14.5 microns to about 17.5 microns. The thickness r₃−r₂ of thedepressed-index cladding region is in the range from 0.5 microns to 12.0microns, or in the range from about 1.0 microns to about 10.0 microns,or in the range from about 1.5 microns to about 9.0 microns, or in therange from about 2.0 microns to about 8.0 microns.

The depressed-index cladding region may be an offset trench design witha trench volume of about 30% Δ-micron² or greater, or about 50%Δ-micron² or greater, or about 70% Δ-micron² or less, or about 30%Δ-micron² or greater and about 70% Δ-micron² or less, or about 50%Δ-micron² or greater and about 70% Δ-micron² or less. Trench volumeslower than the disclosed ranges have reduced bending performance, andtrench volumes higher than the disclosed ranges no longer operate assingle mode fibers.

The offset trench designs disclosed herein provide advantages overtraditional trench designs that are adjacent to the core region. Morespecifically, the offset trench designs disclosed herein reduceconfinement of the fundamental mode and provide improved band loss atlarge bend diameters (e.g., bend diameters >25 mm) for target opticalfiber mode field diameter and cable cutoff characteristics. Furthermore,the trench designs disclosed herein have a depressed index trenchregion, which advantageously confines the intensity profile of thefundamental LP01 mode propagating through the optical fiber, therebyreducing the optical fiber mode field diameter.

Outer Cladding Region

The outer cladding region is comprised of downdoped silica glass that isdoped with fluorine and/or silica glass with voids. The averageconcentration of downdopant in the outer cladding region is greater thanthe average concentration of downdopant in the core region. In someembodiments, the concentration of fluorine in the outer cladding regionis in the range from about 0.50 wt % to about 2.00 wt %, or in the rangefrom about 0.60 wt % to about 1.00 wt %, or in the range from about 0.70wt % to about 0.80 wt %.

The relative refractive index Δ4 or Δ_(4max) of the outer claddingregion is in the range from about −0.20% to about −0.50%, or in therange from about −0.25% to about −0.45%, or in the range from about−0.30% to about −0.40%, or in the range from about −0.33% to about0.37%. The relative refractive index Δ₄ is preferably constant orapproximately constant. As shown in FIG. 5 , the relative refractiveindex Δ₄ may be equal to the relative refractive index Δ₂.

The inner radius of the outer cladding region is r₃ and has the valuesspecified above. The outer radius r₄ is preferably low to minimize thediameter of the glass fiber to facilitate high fiber count in a cable.The outer radius r₄ of the outer cladding region is less than or equalto 65 microns, or less than or equal to 62.5 microns, or less than orequal to 60.0 microns, or less than or equal to 57.5 microns, or lessthan or equal to 55.0 microns, or less than or equal to 52.5 microns, orless than or equal to 50.0 microns, or in the range from 37.5 microns to62.5 microns, or in the range from 40.0 microns to 60.0 microns, or inthe range from 42.5 microns to 57.5 microns, or in the range from 45.0microns to 55.0 microns. Thus, for example, the diameter of the claddingregion (i.e., outer radius r₄ multiplied by 2) is about 130 microns orless, or about 125 microns or less, or about 120 microns or less, orabout 115 microns or less, or about 110 microns or less, or about 105microns or less, or about 100 microns or less, or about 90 microns orless, or about 80 microns or less, or about 75 microns or less. Thethickness r₄−r₃ of the outer cladding region is in the range from about10.0 microns to about 50.0 microns, or in the range from about 15.0microns to about 45.0 microns, or in the range from about 20.0 micronsto about 40.0 microns, or in the range from about 25.0 microns to about35.0 microns.

Optical Fiber Characteristics

The optical fibers according to the embodiments of the presentdisclosure may have a mode field diameter in the range of about 9microns to about 9.5 microns at 1310 nm and in the range of about 10microns to about 10.5 microns at 1550 nm with a cable cutoff of lessthan about 1530 nm. In some embodiments, the cable cutoff is less thanabout 1500 nm, or less than about 1450 nm, or less than about 1400 nm,or less than about 1300 nm, or less than about 1260 nm.

Additionally, optical fibers according to the embodiments of the presentdisclosure may have an effective area at 1550 nm of about 100 micron² orless, or about 90 micron² or less, or about 80 micron² or less, or about70 micron² or less, or in the range of about 70 micron² to about 90micron², or in the range from about 75 micron² to about 85 micron², orabout 80 micron².

The attenuation of the optical fibers disclosed herein is less than orequal to 0.175 dB/km, or less than or equal to 0.170 dB/km, or less thanor equal to 0.165 dB/km, or less than or equal to 0.160 dB/km, or lessthan or equal to 0.155 dB/km, or less than or equal to 0.150 dB/km at awavelength of 1550 nm.

As shown in FIG. 5 , optical fiber 60 provides an exemplary embodimentof an optical fiber with an alkali doped core, a relative refractiveindex Δ₁ of the core region (1) between about −0.3% to about −0.42%, anda core radius (r₁) between about 4 microns and about 6.5 microns.Additionally, an inner cladding region thickness of optical fiber 60 isbetween about 2 microns and about 12 microns. Optical fiber 60 has anoff-set trench design with a trench volume of 54.5% Δ-micron². Thecladding of optical fiber 60 is fluorine-doped and the depressed-indexcladding region has a radius (r₃) of about 17.5 microns. The opticalproperties of optical fiber 60 are shown in Table 1 below.

TABLE 1 Optical Properties of Optical Fiber 60 Mode Field Diameter (at1310 nm)  9.22 microns Mode Field Diameter (at 1550 nm) 10.27 micronsMode Field Diameter (at 1625 nm) 10.61 microns Zero DispersionWavelength 1319 nm Cable Cutoff 1315 nm Trench Volume 54.5% Δ-micron² 15nm Diameter Bend Loss  0.04 dB/turn 20 nm Diameter Bend Loss 0.009dB/turn 30 nm Diameter Bend Loss 0.001 dB/turn

FIG. 6A depicts second and third exemplary embodiments of opticalfibers, 64, 65 with an alkali doped core and a trench volume of greaterthan about 50% Δ-micron², and wherein the cladding is fluorine doped andthe depressed-index cladding region has a radius (r3) of about 17.5microns. As shown in Table 2 below, optical fiber 64 results in a modefield diameter of 9.07 microns at 1310 nm, and optical fiber 65 resultsin a mode field diameter of 9.39 microns at 1310 nm. The opticalproperties of optical fibers 64 and 65 are shown in Table 2 below.

TABLE 2 Optical Properties of Optical Fibers 64 and 65 Optical Fiber 64Optical Fiber 65 Mode Field Diameter (at 1310 nm)  9.07 microns  9.39microns Mode Field Diameter (at 1550 nm) 10.08 microns 10.48 micronsMode Field Diameter (at 1625 nm) 10.41 microns 10.83 microns ZeroDispersion Wavelength 1319 nm 1320 nm Cable Cutoff 1419 nm 1339 nmTrench Volume 55% Δ-micron² 55% Δ-micron² 15 nm Diameter Bend Loss0.0137 dB/turn 0.042 dB/turn 20 nm Diameter Bend Loss 0.0003 dB/turn0.009 dB/turn 30 nm Diameter Bend Loss 0.0002 dB/turn 0.001 dB/turn

FIG. 6B depicts a fourth exemplary embodiment of optical fiber 66 with achlorine doped core having a chlorine concentration of about 1.8 wt %.Optical fiber 66 also comprises an inner cladding, a depressed-indexcladding region, and an outer cladding of silica doped fluorine. Theinner cladding has a fluorine concentration of 0.73 wt %, thedepressed-index cladding region has a fluorine concentration of 1.5 wt%, and the outer cladding has a fluorine concentration of 0.73 wt %.Furthermore, optical fiber 66 has a glass outer diameter of 125 microns,a primary coating outer diameter of 167 microns, and a secondary coatingouter diameter of 200 microns. The optical properties of optical fiber66 are shown in Table 3 below.

TABLE 3 Optical Properties of Optical Fiber 66 Optical Fiber 66Attenuation (at 1310 nm loss) 0.309 dB/km Attenuation (at 1550 nm loss0.175 dB/km Mode Field Diameter (at 1550 nm) 10.02 microns EffectiveArea (at 1550 nm) 78.85 microns² Cable Cutoff 1296 nm 15 nm DiameterBend Loss (at 1550 nm) 0.066 dB/turn 20 nm Diameter Bend Loss (at 1550nm) 0.004 dB/turn 30 nm Diameter Bend Loss (at 1550 nm) 0.001 dB/turn

FIG. 6C depicts a fifth exemplary embodiment of optical fiber 67 with achlorine doped core having a chlorine concentration of about 1.8 wt %.Optical fiber 67 also comprises an inner cladding, a depressed-indexcladding region, and an outer cladding of silica doped fluorine. Theinner cladding has a fluorine concentration of 0.73 wt %, thedepressed-index cladding region has a fluorine concentration of 1.5 wt%, and the outer cladding has a fluorine concentration of 0.73 wt %.Furthermore, optical fiber 67 has a glass outer diameter of 125 microns,a primary coating outer diameter of 167 microns, and a secondary coatingouter diameter of 200 microns. Optical fiber 67 was drawn underconditions such that the during the draw process, the fiber was slowcooled in a high temperature furnace. More specifically, during the drawprocess, optical fiber 67 was slow cooled in a furnace operating at 900°C. for a period of 0.3 seconds. The optical properties of optical fiber67 are shown in Table 4 below.

TABLE 4 Optical Properties of Optical Fiber 67 Optical Fiber 67Attenuation (at 1550 nm loss) 0.175 dB/km Mode Field Diameter (at 1550nm) 10.02 microns Effective Area (at 1550 nm) 78.0 microns² Cable Cutoff1394 nm

FIG. 6D depicts a sixth exemplary embodiment of optical fiber 68 with achlorine doped core having a chlorine concentration of about 1.8 wt %.Optical fiber 68 also comprises an inner cladding, a depressed-indexcladding region, and an outer cladding of silica doped fluorine. Theinner cladding has a fluorine concentration of 0.73 wt %, thedepressed-index cladding region has a fluorine concentration of 1.5 wt%, and the outer cladding has a fluorine concentration of 0.73 wt %.Furthermore, optical fiber 68 has a glass outer diameter of 100 microns,a primary coating outer diameter of 125 microns, and a secondary coatingouter diameter of 160 microns. The optical properties of optical fiber68 are shown in Table 5 below.

TABLE 5 Optical Properties of Optical Fiber 68 Optical Fiber 68Attenuation (at 1310 nm loss)  0.33 dB/km Attenuation (at 1550 nm loss)0.196 dB/km Mode Field Diameter (at 1550 nm) 10.33 microns Cable Cutoff1183 nm 15 nm Diameter Bend Loss (at 1550 nm) 0.047 dB/turn 20 nmDiameter Bend Loss (at 1550 nm)  0.01 dB/turn

FIG. 6E depicts a seventh exemplary embodiment of optical fiber 69 witha chlorine doped core having a chlorine concentration of about 1.8 wt %.Optical fiber 69 also comprises an inner cladding, a depressed-indexcladding region, and an outer cladding of silica doped fluorine. Theinner cladding has a fluorine concentration of 0.73 wt %, thedepressed-index cladding region has a fluorine concentration of 1.4 wt%, and the outer cladding has a fluorine concentration of 0.73 wt %.Furthermore, optical fiber 69 has a glass outer diameter of 100 microns,a primary coating outer diameter of 131 microns, and a secondary coatingouter diameter of 172 microns. The optical properties of optical fiber69 are shown in Table 6 below.

TABLE 6 Optical Properties of Optical Fiber 69 Optical Fiber 69Attenuation (at 1310 nm loss) 0.330 dB/km Attenuation (at 1550 nm loss)0.179 dB/km Attenuation (at 1625 nm loss) 0.192 dB/km Mode FieldDiameter (at 1550 nm) 10.6 microns Cable Cutoff 1308 nm 10 nm DiameterBend Loss (at 1550 nm) 0.674 dB/turn 15 nm Diameter Bend Loss (at 1550nm) 0.071 dB/turn

The off-set trench design of optical fibers 60 and 64-69 provideimproved microbending sensitivity for the smaller diameter fibersdisclosed herein. More specifically, the off-set trench design disclosedherein provides optimized microbending without sacrificing cable cutoffand mode field diameter.

Primary and Secondary Coatings

The transmissivity of light through an optical fiber is highly dependenton the properties of the coatings applied to the glass fiber. Asdiscussed above (and with reference to FIG. 4 ), the coatings typicallyinclude a primary coating 56 and a secondary coating 58, where thesecondary coating surrounds the primary coating and the primary coatingcontacts the glass fiber (which includes a central core regionsurrounded by a cladding region). An optional tertiary layer (e.g. inklayer) surrounds and directly contacts the secondary coating.

Secondary coating 58 may be a harder material (higher Young's modulus)than primary coating 56 and is designed to protect the glass fiber fromdamage caused by abrasion or external forces that arise duringprocessing, handling, and deployment of the optical fiber. Primarycoating 56 may be a softer material (lower Young's modulus) thansecondary coating 58 and is designed to buffer or dissipates stressesthat result from forces applied to the outer surface of the secondarycoating. Dissipation of stresses within the primary coating attenuatesthe stress and minimizes the stress that reaches the glass fiber. Theprimary coating is especially important in dissipating stresses thatarise due to the microbends the optical fiber encounters when deployedin a cable. The microbending stresses transmitted to the glass fiberneed to be minimized because microbending stresses create localperturbations in the refractive index profile of the glass fiber. Thelocal refractive index perturbations lead to intensity losses for thelight transmitted through the glass fiber. By dissipating stresses, theprimary coating minimizes intensity losses caused by microbending.

Coating Examples—Preparation and Measurement Techniques

The properties of primary and secondary coatings, as disclosed herein,were determined using the measurement techniques described below:

Tensile Properties. The curable secondary coating compositions werecured and configured in the form of cured rod samples for measurement ofYoung's modulus, tensile strength at yield, yield strength, andelongation at yield. The cured rods were prepared by injecting thecurable secondary composition into Teflon® tubing having an innerdiameter of about 0.025″. The rod samples were cured using a Fusion Dbulb at a dose of about 2.4 J/cm² (measured over a wavelength range of225-424 nm by a Light Bug model IL390 from International Light). Aftercuring, the Teflon® tubing was stripped away to provide a cured rodsample of the secondary coating composition. The cured rods were allowedto condition for 18-24 hours at 23° C. and 50% relative humidity beforetesting. Young's modulus, tensile strength at break, yield strength, andelongation at yield were measured using a Sintech MTS Tensile Tester ondefect-free rod samples with a gauge length of 51 mm, and a test speedof 250 mm/min. Tensile properties were measured according to ASTMStandard D882-97. The properties were determined as an average of atleast five samples, with defective samples being excluded from theaverage.

In Situ Glass Transition Temperature. In situ T_(g) measurements ofprimary and secondary coatings were performed on fiber tube-off samplesobtained from coated fibers. The coated fibers included a glass fiberhaving a diameter of 125 microns, a primary coating with thickness 32.5microns surrounding and in direct contact with the glass fiber, and asecondary coating with thickness 26.0 microns surrounding and in directcontact with the glass fiber. The glass fiber and primary coating werethe same for all samples measured. The primary coating was formed fromthe reference primary coating composition described below. Samples witha comparative secondary coating and a secondary coating in accordancewith the present disclosure were measured.

The fiber tube-off samples were obtained using the following procedure:a 0.0055″ Miller stripper was clamped down approximately 1 inch from theend of the coated fiber. The one-inch region of fiber was plunged into astream of liquid nitrogen and held in the liquid nitrogen for 3 seconds.The coated fiber was then removed from the stream of liquid nitrogen andquickly stripped to remove the coating. The stripped end of the fiberwas inspected for residual coating. If residual coating remained on theglass fiber, the sample was discarded, and a new sample was prepared.The result of the stripping process was a clean glass fiber and a hollowtube of stripped coating that included intact primary and secondarycoatings. The hollow tube is referred to as a “tube-off sample”. Theglass and primary and secondary coating diameters were measured from theend-face of the unstripped fiber.

In-situ Tg of the tube-off samples was run using a Rheometrics DMTA IVtest instrument at a sample gauge length of 9 to 10 mm. The width,thickness, and length of the tube-off sample were input to the operatingprogram of the test instrument. The tube-off sample was mounted and thencooled to approximately −85° C. Once stable, the temperature ramp wasrun using the following parameters:

-   -   Frequency: 1 Hz    -   Strain: 0.3%    -   Heating Rate: 2° C./min.    -   Final Temperature: 150° C.    -   Initial Static Force=20.0 g    -   Static>Dynamic Force by=10.0%

The in-situ Tg of a coating is defined as the maximum value of tan δ ina plot of tan δ as a function of temperature, where tan δ is defined as:

tan δ=E″/E′

and E″ is the loss modulus, which is proportional to the loss of energyas heat in a cycle of deformation and E′ is the storage or elasticmodulus, which is proportional to the energy stored in a cycle ofdeformation.

The tube-off samples exhibited distinct maxima in the tan δ plot for theprimary and secondary coatings. The maximum at lower temperature (about−50° C.) corresponded to the in-situ Tg for the primary coating and themaximum at higher temperature (above 50° C.) corresponded to the in-situTg for the secondary coating.

In Situ Modulus of Primary Coating. The in situ modulus was measuredusing the following procedure. A six-inch sample of fiber was obtainedand a one-inch section from the center of the fiber was window-strippedand wiped with isopropyl alcohol. The window-stripped fiber was mountedon a sample holder/alignment stage equipped with 10 mm×5 mm rectangularaluminum tabs that were used to affix the fiber. Two tabs were orientedhorizontally and positioned so that the short 5 mm sides were facingeach other and separated by a 5 mm gap. The window-stripped fiber waslaid horizontally on the sample holder across the tabs and over the gapseparating the tabs. The coated end of one side of the window-strippedregion of the fiber was positioned on one tab and extended halfway intothe 5 mm gap between the tabs. The one-inch window-stripped regionextended over the remaining half of the gap and across the opposing tab.After alignment, the sample was removed and a small dot of glue wasapplied to the half of each tab closest to the 5 mm gap. The fiber wasthen returned to position and the alignment stage was raised until theglue just touched the fiber. The coated end was then pulled away fromthe gap and through the glue such that the majority of the 5 mm gapbetween the tabs was occupied by the window-stripped region of thefiber. The portion of the window-stripped region remaining on theopposing tab was in contact with the glue. The very tip of the coatedend was left to extend beyond the tab and into the gap between the tabs.This portion of the coated end was not embedded in the glue and was theobject of the in situ modulus measurement. The glue was allowed to drywith the fiber sample in this configuration to affix the fiber to thetabs. After drying, the length of fiber fixed to each of the tabs wastrimmed to 5 mm. The coated length embedded in glue, the non-embeddedcoated length (the portion extending into the gap between the tabs), andthe primary diameter were measured.

The in situ modulus measurements were performed on a Rheometrics DMTA IVdynamic mechanical testing apparatus at a constant strain of 9e-6 1/sfor a time of forty-five minutes at room temperature (21° C.). The gaugelength was 15 mm. Force and the change in length were recorded and usedto calculate the in situ modulus of the primary coating. The tab-mountedfiber samples were prepared by removing any epoxy from the tabs thatwould interfere with the 15 mm clamping length of the testing apparatusto ensure that there was no contact of the clamps with the fiber andthat the sample was secured squarely to the clamps. The instrument forcewas zeroed out. The tab to which the non-coated end of the fiber wasaffixed was then mounted to the lower clamp (measurement probe) of thetesting apparatus and the tab to which the coated end of the fiber wasaffixed was mounted to the upper (fixed) clamp of the testing apparatus.The test was then executed, and the sample was removed once the analysiswas completed.

In Situ Modulus of Secondary Coating. For secondary coatings, the insitu modulus was measured using fiber tube-off samples prepared from thefiber samples. A 0.0055 inch Miller stripper was clamped downapproximately 1 inch from the end of the fiber sample. This one-inchregion of fiber sample was immersed into a stream of liquid nitrogen andheld for 3 seconds. The fiber sample was then removed and quicklystripped. The stripped end of the fiber sample was then inspected. Ifcoating remained on the glass portion of the fiber sample, the tube-offsample was deemed defective and a new tube-off sample was prepared. Aproper tube-off sample is one that stripped clean from the glass andconsists of a hollow tube with primary and secondary coating. The glassand primary and secondary coating diameters were measured from theend-face of the un-stripped fiber sample.

The fiber tube-off samples were run using a Rheometrics DMTA IVinstrument at a sample gauge length 11 mm to obtain the in situ modulusof the secondary coating. The width, thickness, and length weredetermined and provided as input to the operating software of theinstrument. The sample was mounted and run using a time sweep program atambient temperature (21° C.) using the following parameters:

-   -   Frequency: 1 Rad/sec    -   Strain: 0.3%    -   Total Time=120 sec.    -   Time Per Measurement=1 sec    -   Initial Static Force=15.0 g    -   Static>Dynamic Force by=10.0%        Once completed, the last five E′ (storage modulus) data points        were averaged. Each sample was run three times (fresh sample for        each run) for a total of fifteen data points. The averaged value        of the three runs was reported.

Puncture Resistance of Secondary Coating. Puncture resistancemeasurements were made on samples that included a glass fiber, a primarycoating, and a secondary coating. The glass fiber had a diameter of 125microns. The primary coating was formed from the reference primarycoating composition listed in Table 7 below. Samples with varioussecondary coatings were prepared as described below. The thicknesses ofthe primary coating and secondary coating were adjusted to vary thecross-sectional area of the secondary coating as described below. Theratio of the thickness of the secondary coating to the thickness of theprimary coating was maintained at about 0.8 for all samples.

The puncture resistance was measured using the technique described inthe article entitled “Quantifying the Puncture Resistance of OpticalFiber Coatings”, by G. Scott Glaesemann and Donald A. Clark, publishedin the Proceedings of the 52^(nd) International Wire & Cable Symposium,pp. 237-245 (2003). A summary of the method is provided here. The methodis an indentation method. A 4-centimeter length of optical fiber wasplaced on a 3 mm-thick glass slide. One end of the optical fiber wasattached to a device that permitted rotation of the optical fiber in acontrolled fashion. The optical fiber was examined in transmission under100× magnification and rotated until the secondary coating thickness wasequivalent on both sides of the glass fiber in a direction parallel tothe glass slide. In this position, the thickness of the secondarycoating was equal on both sides of the optical fiber in a directionparallel to the glass slide. The thickness of the secondary coating inthe directions normal to the glass slide and above or below the glassfiber differed from the thickness of the secondary coating in thedirection parallel to the glass slide. One of the thicknesses in thedirection normal to the glass slide was greater and the other of thethicknesses in the direction normal to the glass slide was less than thethickness in the direction parallel to the glass slide. This position ofthe optical fiber was fixed by taping the optical fiber to the glassslide at both ends and is the position of the optical fiber used for theindentation test.

Indentation was carried out using a universal testing machine (Instronmodel 5500R or equivalent). An inverted microscope was placed beneaththe crosshead of the testing machine. The objective of the microscopewas positioned directly beneath a 75° diamond wedge indenter that wasinstalled in the testing machine. The glass slide with taped fiber wasplaced on the microscope stage and positioned directly beneath theindenter such that the width of the indenter wedge was orthogonal to thedirection of the optical fiber. With the optical fiber in place, thediamond wedge was lowered until it contacted the surface of thesecondary coating. The diamond wedge was then driven into the secondarycoating at a rate of 0.1 mm/min and the load on the secondary coatingwas measured. The load on the secondary coating increased as the diamondwedge was driven deeper into the secondary coating until punctureoccurred, at which point a precipitous decrease in load was observed.The indentation load at which puncture was observed was recorded and isreported herein as grams of force (g) and referred to herein as“puncture load”. The experiment was repeated with the optical fiber inthe same orientation to obtain ten measurement points, which wereaveraged to determine a puncture load for the orientation. A second setof ten measurement points was taken by rotating the orientation of theoptical fiber by 180°.

Macrobending Loss. Macrobending loss was determined using the mandrelwrap test specified in standard IEC 60793-1-47. In the mandrel wraptest, the fiber is wrapped one or more times around a cylindricalmandrel having a specified diameter, and the increase in attenuation ata specified wavelength due to the bending is determined. Attenuation inthe mandrel wrap test is expressed in units of dB/turn, where one turnrefers to one revolution of the fiber about the the mandrel.Macrobending losses at a wavelength of 850 nm and 1625 nm weredetermined for selected examples described below with the mandrel wraptest using mandrels with diameters of 15 mm and 30 mm.

Exemplary Embodiments of Primary and Secondary Coatings

The specific properties of primary coating 56 and secondary coating 58may be tailored to provide sufficient robustness and microbendingcharacteristics of the smaller diameter fibers disclosed herein. Forexample, primary coating 56 may have a low Young's modulus and/or a lowin situ modulus. The Young's modulus of the primary coating is less thanor equal to about 0.7 MPa, or less than or equal to about 0.6 MPa, orless than or equal to 0.5 about MPa, or less than or equal to about 0.4MPa, or in the range from about 0.1 MPa to about 0.7 MPa, or in therange from about 0.3 MPa to about 0.6 MPa. The in situ modulus of theprimary coating is less than or equal to about 0.50 MPa, or less than orequal to about 0.30 MPa, or less than or equal to about 0.25 MPa, orless than or equal to about 0.20 MPa, or less than or equal to about0.15 MPa, or less than or equal to about 0.10 MPa, or in the range fromabout 0.05 MPa to about 0.25 MPa, or in the range from about 0.10 MPa toabout 0.20 MPa.

Primary coating 56 preferably has a higher refractive index thancladding region 50 of the glass fiber in order to allow it to striperrant optical signals away from core region 48. Primary coating 56should maintain adequate adhesion to the glass fiber during thermal andhydrolytic aging, yet still be strippable from the glass fiber forsplicing purposes.

To facilitate smaller diameter optical fibers, secondary coating 58 mayhave a smaller thickness compared to traditional cables. However,secondary coating 58 must still maintain the required robustness andpuncture resistance needed for optical fibers. As the thickness of thesecondary coating decreases, its protective function diminishes.Puncture resistance is a measure of the protective function of asecondary coating. A secondary coating with a higher puncture resistancewithstands greater impact without failing and provides better protectionfor the glass fiber.

In order to provide the required robustness and puncture resistance,secondary coating 58 may have an in situ modulus greater than about 1500MPa, or greater than about 1600 MPa, or greater than about 1800 MPa, orgreater than about 2200 MPa, or greater than about 2500 MPa, or greaterthan about 2600 MPa, or greater than about 2700 MPa, or in the rangefrom about 1600 MPa to about 3000 MPa, or in the range from about 1800MPa to about 2800 MPa, or in the range from about 2000 MPa to about 2800MPa, or in the range from about 2400 MPa to about 2800 MPa.

Primary and secondary coatings are typically formed by applying acurable coating composition to the glass fiber as a viscous liquid andcuring. The optical fiber may also include a tertiary coating thatsurrounds the secondary coating. The tertiary coating may includepigments, inks, or other coloring agents to mark the optical fiber foridentification purposes and typically has a Young's modulus similar tothe Young's modulus of the secondary coating.

Secondary coating 58 may be comprised of a trifunctional monomer. Aglass transition temperature (Tg) of secondary coating 58 may be greaterthan about 50° C., or greater than about 60° C., or greater than about70° C., or greater than about 80° C., or greater than about 90° C., orgreater than about 100° C.

Suitable primary coatings 56 and/or secondary coatings 58 may be used sothat optical fiber 46 has a puncture resistance greater than or equal toabout 5 g, or greater than or equal to about 10 g, or greater than orequal to about 15 g, or greater than or equal to about 20 g, or greaterthan or equal to about 25 g, or greater than or equal to about 30 g, orgreater than or equal to about 35 g, or greater than or equal to about40 g, or greater than or equal to about 45 g, or greater than or equalto about 50 g, or greater than or equal to about 55 g, or greater thanor equal to about 60 g.

Reduced Diameter Exemplary Embodiments

As discussed above, the optical fibers of the embodiments disclosedherein may have a glass diameter and/or a coating diameter with reduceddiameters. In some embodiments, cladding region 50 may have an outerdiameter of about 125 microns or less and secondary coating 58 may havean outer diameter of about 210 microns or less. Cladding region 50 mayhave an outer diameter of about 110 microns or less, or about 100microns or less, or about 90 microns or less, or about 80 microns orless. Furthermore, secondary coating 58 may have an outer diameter ofabout 210 microns or less, or about 200 microns or less, or about 180microns or less, or about 170 microns or less, or about 160 microns orless. It is noted that the outer diameter of cladding region 50 is theglass diameter of optical fiber 46 and that the outer diameter ofsecondary coating 58 may be the outer overall diameter of optical fiber46 (when an outer tertiary ink layer is not applied).

In some exemplary examples, cladding region 50 has an outer diameter ofabout 125 microns and secondary coating 58 has an outer diameter ofabout 200 microns or less, or cladding region 50 has an outer diameterof about 125 microns and secondary coating 58 has an outer diameter ofabout 180 microns or less, or cladding region 50 has an outer diameterof about 125 microns and secondary coating 58 has an outer diameter ofabout 170 microns or less, or cladding region 50 has an outer diameterof about 125 microns and secondary coating 58 has an outer diameterbetween about 155 and 175 microns, or cladding region 50 has an outerdiameter of about 125 microns and secondary coating 58 has an outerdiameter between about 160 and 170 microns. In yet other exemplaryembodiments, cladding region 50 has an outer diameter of about 110microns or less and secondary coating 58 has an outer diameter of about200 microns or less, or cladding region 50 has an outer diameter ofabout 90 microns or less and secondary coating 58 has an outer diameterof about 180 microns or less, or cladding region 50 has an outerdiameter of about 90 microns and secondary coating 58 has an outerdiameter between about 155 and 175 microns, or cladding region 50 has anouter diameter of about 90 microns and secondary coating 58 has an outerdiameter between about 160 and 170 microns.

As discussed above, the reduced diameter optical fiber profile designsof the present disclosure provide particular advantages, such as, forexample, a higher fiber count in submarine cables. However, a reductionin the cladding diameter of an optical fiber may allow some light toleak through the cladding, due to the reduced profile of the cladding.Thus, the off-set trench designs of the present disclosure have trenchvolumes of about 30% Δ-micron² or greater to advantageously reduce“tunneling” or “radiation” losses caused by leaking of the light throughthe reduced diameter cladding.

FIG. 7 shows radiation loss as a function of clad diameter for twooptical fibers having a mode field diameter of 9.2 microns at 1310 nmand cable cutoff of 1430 nm. Exemplary optical fiber 72 has a trenchvolume of about 58% Δ-micron², in accordance with the embodiments of thepresent disclosure, while comparison optical fiber 74 has a trenchvolume of only about 8% Δ-micron². As shown in FIG. 7 , exemplaryoptical fiber 72 has lower radiation loss compared with comparisonoptical fiber 74 over the same cladding diameter range. The largertrench volumes disclosed herein advantageously provide reduced radiationloss, thus providing a more efficient optical fiber. Additionally,off-set trench designs having trench volumes of about 30% Δmicron² orgreater also help in reducing microbending loss in reduced clad diameteroptical fibers. Typically, optical fibers with reduced claddingdiameters demonstrate increased microbending sensitivity. But, off-settrench designs having trench volumes of about 30% Δmicron² or greater,as disclosed herein, provide optical fibers with reduced microbendinglosses.

Primary and secondary coatings may also have reduced diameters comparedto the coating geometry of conventional optical fibers. The radius rs ofthe primary coating is less than or equal to about 85.0 microns, or lessthan or equal to about 80.0 microns, or less than or equal to about 75.0microns, or less than or equal to about 70.0 microns. To facilitatedecreases in the diameter of the optical fiber, it is preferable tominimize the thickness r₅−r₄ of the primary coating. The thickness r₅−r₄of the primary coating is less than or equal to about 25.0 microns, orless than or equal to about 20.0 microns, or less than or equal to about15.0 microns, or less than or equal to about 10.0 microns, or in therange from about 5.0 microns to about 25.0 microns, or in the range fromabout 8.0 microns to about 20.0 microns, or in the range from about 10.0microns to about 17.0 microns.

The radius r₆ of the secondary coating is less than or equal to about95.0 microns, or less than or equal to about 90.0 microns, or less thanor equal to about 85.0 microns, or less than or equal to about 80.0microns. It is also preferable to minimize the thickness r₆−r₅ of thesecondary coating. The thickness r₆−r₅ of the secondary coating is lessthan or equal to about 25.0 microns, or less than or equal to about 20.0microns, or less than or equal to about 15.0 microns, or less than orequal to about 10.0 microns, or in the range from about 5.0 microns toabout 25.0 microns, or in the range from about 8.0 microns to about 20.0microns, or in the range from about 10.0 microns to about 18.0 microns,or in the range from about 12.0 microns to about 16.0 microns.

A ratio of the thickness of the secondary coating to the thickness ofthe primary coating may be from about 0.50 to about 1.40, or from about0.60 to about 1.3 or from about 0.65 to about 1.2, or from about 0.70 toabout 1.10, or from about 0.75 to about 1.00, or about 0.80.

Thus, optical fibers in accordance with the embodiments of the presentdisclosure have reduced coating diameters, or reduced glass diameters,or both reduced coating and glass diameters from traditional opticalfibers. Such helps to increase the “fiber count” within, for example, asubmarine cable.

Table 7 below shows an average coating thickness for five secondarycoating samples. Examples 1 and 2 compared with Examples 3, 4, and 5show that average secondary coating thicknesses in the range of 8.0microns to 20.0 microns produced higher tensile strength than averagethicknesses below this range. The higher tensile strength exhibited byExamples 1 and 2 enable use of thinner secondary coatings on opticalfibers, such as those used in submarine cables.

TABLE 7 Thickness of Secondary Coating Example No. 1 2 3 4 5 Average10.2 10.7 6.7 6.7 6.0 Secondary Coating microns microns microns micronsmicrons Thickness Tensile Strength 89% 93% 4% 26% 24% (100 kpsiscreening rate)

Exemplary Primary and Secondary Coatings

Exemplary primary and secondary coatings are discussed below, along withmeasurements of strength and puncture resistance of the coatings.

Primary Coating—Composition. The primary coating composition includesthe formulation given in Table 8 below and is typical of commerciallyavailable primary coating compositions.

TABLE 8 Reference Primary Coating Composition Component AmountOligomeric Material 50.0 wt % SR504 46.5 wt % NVC  2.0 wt % TPO  1.5 wt% Irganox 1035  1.0 pph 3-Acryloxypropyl trimethoxysilane  0.8 pphPentaerythritol tetrakis(3-mercapto 0.032 pph propionate

Where the oligomeric material was prepared as described herein fromH12MDI, HEA, and PPG4000 using a molar ratio n:m:p=3.5:3.0:2.0, SR504 isethoxylated(4)nonylphenol acrylate (available from Sartomer), NVC isN-vinylcaprolactam (available from Aldrich), TPO (a photoinitiator) is(2,4,6-trimethylbenzoyl)-diphenyl phosphine oxide (available from BASF),Irganox 1035 (an antioxidant) is benzenepropanoic acid,3,5-bis(1,1-dimethylethyl)-4-hydroxythiodi-2,1-ethanediyl ester(available from BASF), 3-acryloxypropyl trimethoxysilane is an adhesionpromoter (available from Gelest), and pentaerythritoltetrakis(3-mercaptopropionate) (also known as tetrathiol, available fromAldrich) is a chain transfer agent. The concentration unit “pph” refersto an amount relative to a base composition that includes all monomers,oligomers, and photoinitiators. For example, a concentration of 1.0 pphfor Irganox 1035 corresponds to 1 g Irganox 1035 per 100 g combined ofoligomeric material, SR504, NVC, and TPO.

The oligomeric material was prepared by mixing H12MDI (4,4′-methylenebis(cyclohexyl isocyanate)), dibutyltin dilaurate and2,6-di-tert-butyl-4 methylphenol at room temperature in a 500 mL flask.The 500 mL flask was equipped with a thermometer, a CaCl2 drying tube,and a stirrer. While continuously stirring the contents of the flask,PPG4000 was added over a time period of 30-40 minutes using an additionfunnel. The internal temperature of the reaction mixture was monitoredas the PPG4000 was added and the introduction of PPG4000 was controlledto prevent excess heating (arising from the exothermic nature of thereaction). After the PPG4000 was added, the reaction mixture was heatedin an oil bath at about 70° C. to 75° C. for about 1 to 1½ hours. Atvarious intervals, samples of the reaction mixture were retrieved foranalysis by infrared spectroscopy (FTIR) to monitor the progress of thereaction by determining the concentration of unreacted isocyanategroups. The concentration of unreacted isocyanate groups was assessedbased on the intensity of a characteristic isocyanate stretching modenear 2265 cm⁻¹. The flask was removed from the oil bath and its contentswere allowed to cool to below 65° C. Addition of supplemental HEA wasconducted to insure complete quenching of isocyanate groups. Thesupplemental HEA was added dropwise over 2-5 minutes using an additionfunnel. After addition of the supplemental HEA, the flask was returnedto the oil bath and its contents were again heated to about 70° C. to75° C. for about 1 to 1½ hours. FTIR analysis was conducted on thereaction mixture to assess the presence of isocyanate groups and theprocess was repeated until enough supplemental HEA was added to fullyreact any unreacted isocyanate groups. The reaction was deemed completewhen no appreciable isocyanate stretching intensity was detected in theFTIR measurement.

Secondary Coating—Compositions. Four curable secondary coatingcompositions (A, SB, SC, and SD) are listed in Table 9.

TABLE 9 Secondary Coating Compositions Composition Component A SB SC SDPE210 (wt %) 15.0 15.0 15.0 15.0 M240 (wt %) 72.0 72.0 72.0 62.0 M2300(wt %) 10.0 — — — M3130 (wt %) — 10.0 — — M370 (wt %) — — 10.0 20.0 TPO(wt %) 1.5 1.5 1.5 1.5 Irgacure 184 (wt %) 1.5 1.5 1.5 1.5 Irganox 1035(pph) 0.5 0.5 0.5 0.5 DC-190 (pph) 1.0 1.0 1.0 1.0PE210 is bisphenol-A epoxy diacrylate (available from Miwon SpecialtyChemical, Korea), M240 is ethoxylated (4) bisphenol-A diacrylate(available from Miwon Specialty Chemical, Korea), M2300 is ethoxylated(30) bisphenol-A diacrylate (available from Miwon Specialty Chemical,Korea), M3130 is ethoxylated (3) trimethylolpropane triacrylate(available from Miwon Specialty Chemical, Korea), TPO (a photoinitiator)is (2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (available fromBASF), Irgacure 184 (a photoinitiator) is 1-hydroxycyclohexyl-phenylketone (available from BASF), Irganox 1035 (an antioxidant) isbenzenepropanoic acid,3,5-bis(1,1-dimethylethyl)-4-hydroxythiodi-2,1-ethanediyl ester(available from BASF). DC190 (a slip agent) is silicone-ethyleneoxide/propylene oxide copolymer (available from Dow Chemical). Theconcentration unit “pph” refers to an amount relative to a basecomposition that includes all monomers and photoinitiators. For example,for secondary coating composition A, a concentration of 1.0 pph forDC-190 corresponds to 1 g DC-190 per 100 g combined of PE210, M240,M2300, TPO, and Irgacure 184.

Secondary Coatings—Tensile Properties. The Young's modulus, tensilestrength at yield, yield strength, and elongation at yield of secondarycoatings made from secondary compositions A, SB, SC, and SD weremeasured using the technique described above. The results are summarizedin Table 10.

TABLE 10 Tensile Properties of Secondary Coatings Secondary CompositionProperty A SB SC SD Young's 2049.08 2531.89 2652.51 2775.94 Modulus(MPa) Tensile 86.09 75.56 82.02 86.08 Strength (MPa) Yield 48.21 61.2366.37 70.05 Strength (MPa) Elongation 4.60 4.53 4.76 4.87 at Yield (%)Fracture 0.8580 0.8801 0.9471 0.9016 Toughness, K_(c) (MPa*m^(1/2))

The results show that secondary coatings prepared from compositions SB,SC, and SD exhibited higher Young's modulus and higher yield strengththan the secondary coating prepared from comparative composition A.Additionally, the secondary coatings prepared from compositions SB, SC,and SD exhibited higher fracture toughness than the secondary coatingprepared from composition A. The higher values exhibited by compositionSB, SC, and SD enable use of thinner secondary coatings on opticalfibers without sacrificing performance. As discussed above, thinnersecondary coatings reduce the overall diameter of the optical fiber andprovide higher fiber counts in cables of a given cross-sectional area(such as submarine cables).

Exemplary Optical Fiber Embodiments

The experimental examples and principles disclosed herein indicate thatsufficient microbending and puncture resistance properties can beachieved in a reduced diameter optical fiber by tailoring the coatingproperties of the optical fiber. More specifically, the higher modulusof the secondary coating provides sufficient puncture resistance for thereduced diameter optical profile. The above-disclosed thickness ratio ofthe secondary coating to the primary coating provides a reduced diameteroptical fiber without sacrificing puncture resistance. Furthermore, theexperimental examples and principles disclosed herein indicate thatsufficient attenuation can be achieved in the reduced diameter opticalfiber by providing an alkali doped core.

To examine the effect of the thickness and modulus of the primary andsecondary coatings on transmission of a radial force to a glass fiber, aseries of modeled examples was considered. In the model, a radialexternal load P was applied to the surface of the secondary coating ofan optical fiber and the resulting load at the surface of the glassfiber was calculated. The glass fiber was modeled with a Young's modulusof 73.1 GPa (consistent with silica glass). The Poisson ratios v_(p) andv_(s) of the primary and secondary coatings were fixed at 0.48 and 0.33,respectively. A comparative sample C1 and six samples M1 -M6 inaccordance with the present disclosure were considered. The comparativesample included primary and secondary coatings with thicknesses andmoduli consistent with optical fibers known in the art. Samples M1-M6are examples with reduced thicknesses of the primary and secondarycoatings, in accordance with the embodiments of the present disclosure.Parameters describing the configurations of the primary and secondarycoatings are summarized in Table 11.

TABLE 11 Coating Properties of Modeled Optical Fibers Glass PrimaryCoating Secondary Coating Core In Situ Young's Diameter Modulus DiameterThickness Modulus Diameter Thickness Sample (μm) (MPa) (μm) (μm) (MPa)(μm) (μm) C1 125 0.20 190 32.5 1600 242 26.0 M1 125 0.14 167 21.0 1900200 16.5 M2 125 0.12 161 18.0 1900 190 14.5 M3 125 0.10 155 15.0 2000180 12.5 M4 125 0.09 150 12.5 2300 170 10.0 M5 115 0.12 145 15.0 2200170 12.5 M6 110 0.11 138 14.0 2200 160 11.0

Table 12 below summarizes the load P1 at the outer surface of the glassfiber as a fraction of load P applied to the surface of the secondarycoating. The ratio P1/P is referred to herein as the load transferparameter and corresponds to the fraction of external load P transmittedthrough the primary and secondary coatings to the surface of the glassfiber. The load P is a radial load and the load transfer parameter P1/Pwas calculated from a model based on Eqs. (9)-(11):

$\begin{matrix}{\frac{P_{1}}{P} = \frac{4\left( {1 - v_{p}} \right)\left( {1 - v_{s}} \right)}{\left\{ {A + B} \right\}}} & (9)\end{matrix}$ where $\begin{matrix}{A = \left( \frac{{E_{s}\left( {1 + v_{p}} \right)}\left( {1 - {2v_{p}}} \right)\left( {1 - \left( {r_{4}/r_{5}} \right)^{2}} \right)\left( {1 - \left( {r_{5}/r_{6}} \right)^{2}} \right)}{E_{p}\left( {1 + v_{s}} \right)} \right)} & (10)\end{matrix}$ and $\begin{matrix}{B = \left( {\left( {1 - {2{v_{p}\left( {r_{4}/r_{5}} \right)}^{2}} + \left( {r_{4}/r_{5}} \right)^{2}} \right)\left( {1 - {2{v_{s}\left( {r_{5}/r_{6}} \right)}^{2}} + \left( {r_{5}/r_{6}} \right)^{2}} \right)} \right)} & (11)\end{matrix}$

In Eqs. (9)-(11), v_(p) and v_(s) are the Poisson's ratios of theprimary and secondary coatings, r₄ is the outer radius of the glassfiber, r₅ is the outer radius of the primary coating, r₆ is the outerradius of the secondary coating, E_(p) is the in situ modulus of theprimary coating, and E_(s) is the Young's modulus of the secondarycoating. The scaled load transfer parameter P1/P (scaled) in Table 11corresponds to the ratio P1/P for each sample relative to comparativesample C1.

TABLE 12 Load Transfer Parameter (P1/P) at Surface of Glass Fiber SampleP1/P P1/P (scaled) C1 0.004440 1.00 M1 0.004286 0.97 M2 0.004369 0.98 M30.0042491 0.97 M4 0.004240 0.95 M5 0.004184 0.94 M6 0.004153 0.94

The modeled examples show that despite smaller coating thicknesses,optical fibers having primary and secondary coatings as described hereinexhibit a reduction in the force experienced by a glass fiber relativeto a comparative optical fiber having conventional primary and secondarycoatings with conventional thicknesses. The resulting reduction inoverall size of the optical fibers described herein enables higher fibercount in cables of a given size (or smaller cable diameters for a givenfiber count) without increasing the risk of damage to the glass fibercaused by external forces.

The scaled load transfer parameter P₁/P (scaled) of the secondarycoating is less than about 0.99, or less than about 0.97, or less thanabout 0.95. The load transfer parameter P₁/P of the secondary coating isless than about 0.005, or less than 0.0045, or less than about 0.00445,or less than about 0.00444, or less than about 0.0043, or less thanabout 0.0042, or less than about 0.0041 or in the range from about 0.005to about 0.0041, or in the range from about 0.0045 to about 0.0042, orin the range from about 0.00445 to about 0.00420, or in the range fromabout 0.00440 to about 0.004200.

Table 13 below provides additional modeled examples in accordance withembodiments of the present disclosure. Samples M7-M18 are examples withreduced thicknesses of the primary and secondary coatings. Parametersdescribing the configurations of the primary and secondary coatings aresummarized in Table 12.

TABLE 13 Coating Properties of Modeled Optical Fibers Load TransferGlass Primary Coating Secondary Coating Parameter (P1/P) at Core In SituYoung's Surface of Glass Fiber Diameter Modulus Diameter ModulusDiameter P1/P Sample (μm) (MPa) (μm) (MPa) (μm) P1/P (scaled) M7 1150.18 162 1900 200 0.00430 0.97 M8 115 0.16 157 2000 190 0.00421 0.95 M9115 0.14 151 2000 180 0.00436 0.98 M10 115 0.11 146 2000 170 0.004280.96 M11 110 0.2 160 1900 200 0.00429 0.97 M12 110 0.19 154 2000 1900.00437 0.99 M13 110 0.16 149 2000 180 0.00432 0.97 M14 110 0.13 1432000 170 0.00422 0.95 M15 105 0.22 158 1900 200 0.00427 0.96 M16 1050.20 152 2000 190 0.00411 0.93 M17 105 0.18 147 2000 180 0.00427 0.96M18 105 0.16 141 2000 170 0.00445 1.00

Fiber Draw Process

The optical fibers disclosed herein may be formed from a continuousoptical fiber manufacturing process, during which a glass fiber is drawnfrom a heated preform and sized to a target diameter. The glass fiber isthen cooled and directed to a coating system that applies a liquidprimary coating composition to the glass fiber. Two process options areviable after application of the liquid primary coating composition tothe glass fiber. In one process option (wet-on-dry process), the liquidprimary coating composition is cured to form a solidified primarycoating, the liquid secondary coating composition is applied to thecured primary coating, and the liquid secondary coating composition iscured to form a solidified secondary coating. In a second process option(wet-on-wet process), the liquid secondary coating composition isapplied to the liquid primary coating composition, and both liquidcoating compositions are cured simultaneously to provide solidifiedprimary and secondary coatings. After the fiber exits the coatingsystem, the fiber is collected and stored at room temperature.Collection of the fiber typically entails winding the fiber on a spooland storing the spool.

In some processes, the coating system further applies a tertiary coatingcomposition to the secondary coating and cures the tertiary coatingcomposition to form a solidified tertiary coating. Typically, thetertiary coating is an ink layer used to mark the fiber foridentification purposes and has a composition that includes a pigmentand is otherwise similar to the secondary coating. The tertiary coatingis applied to the secondary coating and cured. The secondary coating hastypically been cured at the time of application of the tertiary coating.The primary, secondary, and tertiary coating compositions can be appliedand cured in a common continuous manufacturing process. Alternatively,the primary and secondary coating compositions are applied and cured ina common continuous manufacturing process, the coated fiber iscollected, and the tertiary coating composition is applied and cured ina separate offline process to form the tertiary coating.

Multicore Optical Fibers

The optical fibers disclosed herein may be used in a multicore opticalfiber design. With the optical fibers of the present disclosure, such amulticore optical fiber design can be achieved that has a maximum numberof cores in a smaller profile cable, while maintaining low cross talkbetween the fibers, low tunneling loss from corner fibers to the edge,and good bending performance. For example, a multicore optical fiber mayinclude the small profile fibers disclosed herein, each having a halogenand/or an alkali metal doped core and an offset trench design asdiscussed above. Thus, the above-discussed single-core optical fibersmay be used in a multicore optical fiber to provide the same mode fielddiameter, effective area, and attenuation as discussed above.

FIG. 8A shows a cross-sectional view of an exemplary multicore opticalfiber 80 with a circular profile. As shown in FIG. 8A, multicore opticalfiber 80 includes a central fiber axis 85 (the centerline of multicoreoptical fiber 80), a plurality of cores 90, and a cladding matrix 94that forms a common cladding 98. Cores 90 are disposed within claddingmatrix 94, with each core 90 forming a core fiber CF₁, CF₂ thatgenerally extends through a length of multicore optical fiber 80parallel to central fiber axis 85.

Each core 90 includes a central axis or centerline CL₁ and CL₂ and anouter radius r₁ and r₂, respectively. It is noted that outer radius r₁,r₂ are similar to radius r₁, as disclosed above with reference to FIG. 5. As shown in FIG. 8A, a position of each of the centerlines CL₁ and CL₂within multicore optical fiber 80 can be defined using Cartesiancoordinates with central fiber axis 85 defining the origin (0, 0) of anx-y coordinate system coincident with the coordinate system defined bythe radial coordinate R. The position of centerline CL₁ can be definedas (x₁, y₁) and the position of centerline CL₂ can be defined as (x₂,y₂). A distance D_(c1)-c₂ between centerlines CL₁ and CL₂ can then bedefined as √[(x₂−x₁)²+(y₂−y₁)²]. Thus, for a given core 90 having acenterline CL_(i) and an adjacent core 90 having a centerline CL_(j), adistance D_(Ci-Cj) is defined as √[(x_(j)−x_(i))²+(y_(j)−y_(i))²], asalso discussed further below.

Cores 90 may be similar to and comprise the same materials andproperties as core region 48 (as discussed above with reference to thesingle-core optical fibers), and cladding matrix 94 may be similar toand comprise the same materials and properties as cladding region 50 (asalso discussed above with reference to the single-core optical fibers).Thus, each core 90 of multicore optical fiber 80 may be surrounded by acladding region with an off-set trench design, as discussed above.According to embodiments of the present disclosure, a multicore opticalfiber includes a first core, a first inner cladding surrounding thefirst core, a second core, a second inner cladding surrounding thesecond core, and a common cladding surrounding the first core and thesecond core. Additionally, a primary coating, a secondary coating, andoptionally a tertiary coating may be disposed on cladding matrix 94, asalso discussed above.

Although FIG. 8A only shows two cores 90, multicore optical fiber 80 maycomprise more than two cores, as also discussed below. Furthermore,multicore optical fiber 80 may comprise a circular cross-sectionalprofile (as shown in FIG. 8A) or a rectangular ribbon cross-sectionalprofile. FIG. 8B shows additional exemplary embodiments of multicoreoptical fiber 80.

Multicore optical fiber 80 may comprise any number of cores 90 in anyconfiguration as is known in the art. For example, the total number ofcores 90 may be from 2 to 20, 2 to 18, 2 to 16, 2 to 12, 2 to 10, 2 to8, 2 to 6, 2 to 4, 2 to 3, 4 to 20, 4 to 18, 4 to 16, 4 to 12, 4 to 10,4 to 8, 4 to 6, 6 to 20, 6 to 18, 6 to 16, 6 to 12, 6 to 10, 6 to 8, 8to 20, 8 to 18, 8 to 16, 8 to 12, 8 to 10, 10 to 20, 10 to 18, 10 to 16,10 to 12, 12 to 20, 12 to 18, or 12 to 16. For example, the total numberof cores 90 can be 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20, or any total number of cores between any of thesevalues. The total number of cores 90 can be even or odd and can bearranged in any pattern within cladding matrix 94, non-limiting examplesof which include a square pattern, a rectangular pattern, a circularpattern, and a hexagonal lattice pattern. FIG. 9 , for example, shows amulticore optical fiber comprising 4 cores arranged in a square pattern.

As shown in FIG. 8C, one or more of the plurality of cores 90 ofmulticore optical fiber 80 is surrounded by an inner cladding 95 and anouter cladding 97, such that outer cladding 97 surrounds inner cladding95 between inner cladding 95 and common cladding 98. Each inner cladding95 has an outer radius no and an inner radius that corresponds to theouter radius r₁ of core 90. Inner cladding 95 has a width δr_(IC1)defined by the outer radius r₁ of core 90 and the outer radius no ofinner cladding 95. Core 90 can have a diameter d corresponding to 2*r₁,and inner cladding 95 can have a diameter dice corresponding to2*r_(IC1). Each outer cladding 97 has an outer radius r_(OC1) and aninner radius that corresponds to the outer radius r_(IC1) of innercladding 95. Outer cladding 97 has a width δr_(OC1) defined by the outerradius r_(IC1) of inner cladding 95 and the outer radius δr_(OC1) ofouter cladding 97. It is noted that outer radius r_(IC1) of innercladding 95 is similar to radius r₂ and that outer radius r_(OC1) ofouter cladding 97 is similar to radius r₃, as discussed above withreference to FIG. 5 . Common cladding 98 has an outer radius R_(oc), asshown in FIG. 8A.

Inner cladding 95 may be similar to and comprise the same materials andproperties as inner cladding region 51 (as discussed above withreference to the single-core optical fibers), outer cladding 97 may besimilar to and comprise the same materials and properties asdepressed-index cladding region 53 (as discussed above with reference tothe single-core optical fibers), and common cladding 98 may be similarto and comprise the same materials and properties as outer claddingregion 55 (as discussed above with reference to the single-core opticalfibers). Thus, for example, outer cladding 97 may form a trench region,similar to depressed-index cladding region 53.

With reference to FIGS. 8A-8C, cores 90 each have a diameter “d” (r₁×2or r₂×2) in a range from about 4 microns to about 20 microns, or about 5microns to about 18 microns, or about 6 microns to about 16 microns, orfrom about 7 microns to about 14 microns, or from about 8 microns toabout 12 microns, or from about 9 microns to about 12 microns. Thediameter d of each core 90 may be the same or different from one or moreother cores 90 in multicore optical fiber 80. The spacing between eachcore 90 D may be constant between each core 90 and may be about 20microns or greater, or about 25 microns or greater, or about 30 micronsor greater, or about 35 microns or greater. Additionally oralternatively, the spacing D may be about 50 microns or less, or about45 microns or less, or about 40 microns or less, or about 35 microns orless. In some embodiments, the spacing D is in the range from about 20microns to about 40 microns, or about 25 microns to about 35 microns, orabout 35 microns. In yet other embodiments, the spacing between two ormore cores 90 may be different from the spacing between two or moreother cores 90. The spacing between cores 90 should be sufficient toreduce cross-talk between the cores, as discussed further below.

When in ribbon form, multicore optical fiber 80 has a cross-sectionalwidth W and a thickness TH, as shown in FIG. 8B. Cores 90 can bearranged in one or more rows along the thickness TH and in one orcolumns extending along the width W. The width W may be about 0.5 mm toabout 3 mm, or about 1 mm to about 2.5 mm, or about 1 mm to about 2 mm.The thickness may about 0.1 to about 0.5 mm, or about 0.2 to 0.4 mm. Inone embodiment, multicore optical fiber 80 has a rectangular ribboncross-sectional profile, comprises 8 cores, has a width W of about 2 mm,and a thickness TH of about 0.3 mm.

In the circular cross-sectional design, the width W of cladding matrix94 is the diameter of common cladding 98 (R_(oc)×2) and may be about 200microns or less, or about 150 microns or less, or about 125 microns orless, or about 80 microns or greater, or in the range of about 80microns to about 125 microns, or about 120 microns to about 130 microns,or about 125 microns.

In yet other embodiments, the diameter of common cladding 98 may beabout 140 microns or greater, or about 150 microns or greater, or about160 microns or greater, or about 170 microns or greater, or about 180microns or greater, or about 190 microns or greater. Additionally oralternatively, the diameter of common cladding 98 may be about 200microns or less, or about 190 microns or less, or about 180 microns orless, or about 170 microns or less, or about 160 microns or less, orabout 150 microns or less, or about 140 microns or less. In someexamples, the diameter of common cladding 98 is in a range from about120 microns to about 140 microns.

FIG. 9 shows an exemplary multicore optical fiber 100 with 4 cores 90arranged in a square design. As shown in FIG. 9 , a distance D1 betweena centerline (CL₁) of a first core and a centerline (CL₂) of an adjacentcore is less than about 50 microns, as measured using a Cartesiancoordinate system, as discussed above. For example, a distance D1between adjacent cores can be greater than about 20 microns, greaterthan about 25 microns, greater than about 28 microns, greater than about30 microns, greater than about 35 microns, or greater than about 40microns. Additionally or alternatively, distance D1 can be less thanabout 45 microns, less than about 40 microns, or less than about 35microns. For example, distance D1 can be from about 20 microns to about50 microns, about 20 microns to about 45 microns, about 20 microns toabout 30 microns, about 28 microns to about 50 microns, about 28 micronsto about 40 microns, about 28 microns to about 30 microns, about 30microns to about 50 microns, about 30 microns to about 40 microns, orabout 40 microns to about 45 microns. Distance D1 between adjacent cores90 can be the same or different for each of the cores.

The distance D2 between two cores separated by a maximum distance in,for example, a 4×4 square pattern, can be about 20 microns or greater,about 25 microns or greater, about 30 microns or greater, about 35microns or greater, or about 40 microns or greater. Additionally oralternatively, distance D2 can be about 50 microns or less, about 45microns or less, about 40 microns or less, about 35 microns or less, orabout 30 microns or less.

As also shown in FIG. 9 , a distance D3 between a centerline of a core90 and an outer radius of common cladding 98 can be about 40 microns orless, about 35 microns or less, about 30 microns or less, about 25microns or less, or about 20 microns or less. Additionally oralternatively, distance D3 can be about 25 microns or greater, about 30microns or greater, about 35 microns or greater, about 35 microns orgreater, or about 40 microns or greater. In some examples, distance D3is in a range from about 25 microns to about 40 microns, or from about30 microns to about 35 microns. It is also contemplated that distance D3may be the same or different for each core 90.

A distance D4 between an outer radius of a core 90 and an outer radiusof common cladding 98 can be about 25 microns or less, about 20 micronsor less, about 18 microns or less, about 16 microns or less, about 14microns or less, about 13 microns or less, about 12 microns or less,about 10 microns or less, about 8 microns or less, or about 6 microns orless. Additionally or alternatively, distance D4 can be about 8 micronsor greater, about 10 microns or greater, about 13 microns or greater,about 16 microns or greater, or about 18 microns or greater. In someexamples, distance D4 is in a range from about 10 microns to about 20μm, or from about 12 microns to about 16 microns. It is alsocontemplated that distance D4 may be the same or different for each core90.

Furthermore, distance D5, as shown in FIG. 9 , may be a radius (e.g.,r₁, r₂) of each core 90. Distance D5 may be in range from about 2microns to about 30 microns, or from about 2.5 microns to about 22.5microns, or from about 5 microns to about 20 microns, or from about 7microns to about 14 microns, or from about 8 microns to about 12microns, or from about 9 microns to about 11 microns. For example, D5may be about 9 microns, about 10 microns, about 10.5 microns, about 11microns, about 11.5 microns, about 12 microns, about 12.5 microns, about13 microns, about 14 microns, about 15 microns, about 15.5 microns,about 16 microns, about 16.5 microns, about 17 microns, about 17.5microns, or about 18 microns. It is also contemplated that distance D5may be the same or different for each core 90.

In one embodiment, the exemplary multicore optical fiber 100 comprises 4core regions in a 4×4 square design such that that the width W of thefiber is about 125 microns, distance D1 is about 45 microns, distance D2is about 63.6 microns, distance D3 is about 30.7 microns, distance D4 isabout 13 microns, and distance D5 is about 17.6 microns.

As discussed above, cores 90 have reduced crosstalk to ensure goodsystem performance. Crosstalk depends on the distance between the coresand the fiber length. The average crosstalk between the cores may becomputed from Eq. (12)

X=2κ²LL_(c)  (12)

where k is the coupling coefficient, L is the fiber length, and L_(c) isthe correlation length that depends on the fiber uniformity anddeployment conditions.

In some embodiments, the average crosstalk between adjacent cores 90 isequal to or less than −20 dB, or equal to or less than about −30 db, orequal to or less than about −35 dB, or less than about −40 dB, or lessthan about −45 dB, or less than about −50 dB, or equal to or less thanabout −55 dB, or equal to or less than about −60 dB, as measured for a100 km length of the multicore optical fiber operating at 1550 nm.

FIG. 10 shows a plot of crosstalk and spacing between adjacent cores 90in multicore optical fiber designs along a fiber length of 100 km. Trace200 represents a multicore optical fiber design with a step indexprofile and an effective area of 80 micron², trace 210 represents amulticore optical fiber design with an offset trench design (asdisclosed herein) and with an effective area of 80 micron², and trace220 represents a multicore optical fiber design with an offset trenchdesign (as disclosed herein) and an effective area of 100 micron². Asshown in FIG. 10 , trace 220 has lower crosstalk than trace 200 as thespacing between the cores increases. Furthermore, trace 210 has lowercrosstalk than trace 220 as the spacing between the cores increases.Thus, FIG. 10 shows that a multicore design with cores each having aneffective area of 80 micron² and an offset trench design advantageouslyprovides lower crosstalk than cores with an effective area of 100micron² or with fibers having a step index profile.

In the multicore optical fiber designs disclosed herein, cores 90 mayhave an attenuation at 1550 nm of less than or equal to 0.18 dB/km, orless than or equal to 0.175 dB/km, or less than or equal to 0.170 dB/km,or less than or equal to 0.165 dB/km, or less than or equal to 0.160dB/km, or less than or equal to 0.155 dB/km, or less than or equal to0.150 dB/km, as also discussed above. Each core 90 may have the same ordifferent attenuation.

Cores 90 of the multicore optical fibers disclosed herein may have atheoretical cutoff wavelength of less than about 1500 nm, less thanabout 1400 nm, less than about 1300 nm, less than about 1260 nm, or lessthan about 1200 nm. For example, the theoretical cutoff wavelength canbe from about 1300 nm to about 1500 nm or about 1300 nm to about 1400nm. For example, the theoretical cutoff wavelength can be about 1300 nm,about 1310 nm, about 1320 nm, about 1329 nm, about 1330 nm, about 1340nm, about 1350 nm, about 1360 nm, about 1370 nm, about 1380 nm, about1400 nm, about 1500 nm, or any theoretical cutoff wavelength betweenthese values. Each core 90 may have the same or different theoreticalcutoff wavelength.

According to one aspect, a cable cutoff wavelength of cores 90 is lessthan about 1500 nm, less than about 1400 nm, less than about 1300 nm,less than about 1260 nm, or less than about 1200 nm. For example, thecable cutoff wavelength can be from about 1200 nm to about 1500 nm,about 1200 nm to about 1400 nm, about 1200 nm to about 1300 nm, about1300 nm to about 1500 nm, about 1300 nm to about 1400 nm, or about 1400nm to about 1500 nm. For example, the cable cutoff wavelength can beabout 1200 nm, about 1209 nm, about 1210 nm, about 1220 nm, about 1230nm, about 1240 nm, about 1250 nm, about 1260 nm, about 1300 nm, about1310 nm, about 1350 nm, about 1400 nm, about 1410 nm, about 1420 nm,about 1430 nm, about 1440 nm, about 1450 nm, about 1460 nm, about 1500nm, or any cable cutoff wavelength between these values. Each coreregion 90 may have the same or different cable cutoff wavelength.

According to one aspect, cores 90 can have a zero dispersion wavelengthfrom about 1280 nm to about 1340 nm. For example, the zero dispersionwavelength can be from about 1290 nm to about 1330 nm, about 1295 nm toabout 1325 nm, about 1300 nm to about 1320 nm, or from about 1305 nm toabout 1315 nm. For example, the zero dispersion wavelength can be about1280 nm, about 1285 nm, about 1289 nm, about 1290 nm, about 1300 nm,about 1301 nm, about 1305 nm, about 1306 nm, about 1310 nm, about 1315nm, or about 1320 nm, or any zero dispersion wavelength between thesevalues. Each core 90 may have the same or different zero dispersionwavelength.

According to an aspect of the present disclosure, cores 90 can have adispersion having an absolute value at 1310 nm of less than 3 ps/nm/kmand a dispersion slope at 1310 nm of less than 0.1 ps/nm^(2/)km. Eachcore 90 may have the same or different dispersion and dispersion slopeat 1310 nm. For example, the absolute value of the dispersion at 1310 nmcan be from about 0.3 ps/nm/km to about 3 ps/nm/km, about 0.3 ps/nm/kmto about 2.5 ps/nm/km, about 0.3 ps/nm/km to about 2.25 ps/nm/km, about0.3 ps/nm/km to about 2 ps/nm/km, about 0.3 ps/nm/km to about 1.75ps/nm/km, about 0.3 ps/nm/km to about 1.5 ps/nm/km, or about 0.3ps/nm/km to about 1 ps/nm/km. For example, the absolute value of thedispersion at 1310 can be about 0.3 ps/nm/km, about 0.35 ps/nm/km, about0.4 ps/nm/km, about 0.5 ps/nm/km, about 0.75 ps/nm/km, about 1 ps/nm/km,about 1.25 ps/nm/km, about 1.5 ps/nm/km, about 1.75 ps/nm/km, about 2ps/nm/km, about 2.25 ps/nm/km, about 2.5 ps/nm/km, about 2.75 ps/nm/km,about 3 ps/nm/km, or any value between these values. In one example, thedispersion slope at 1310 nm can be about 0.075 ps/nm^(2/)km to about 0.1ps/nm^(2/)km, about 0.08 ps/nm^(2/)km to about 0.1 ps/nm²/km, about0.085 ps/nm^(2/)km to about 0.1 ps/nm^(2/)km, about 0.09 ps/nm^(2/)km toabout 0.1 ps/nm^(2/)km, about 0.075 ps/nm^(2/)km to about 0.09ps/nm^(2/)km, about 0.08 ps/nm^(2/)km to about 0.09 ps/nm^(2/)km, orabout 0.085 ps/nm^(2/)km to about 0.09 ps/nm^(2/)km. For example, thedispersion slope at 1310 nm can be about 0.075 ps/nm2/km, about 0.08ps/nm^(2/)km, about 0.085 ps/nm^(2/)km, about 0.086 ps/nm^(2/)km, about0.087 ps/nm^(2/)km, about 0.088 ps/nm^(2/)km, about 0.089 ps/nm^(2/)km,about 0.09 ps/nm^(2/)km, about 0.01 ps/nm^(2/)km, or any value betweenthese values.

According to an aspect of the present disclosure, cores 90 can have adispersion at 1550 nm of less than 22 ps/nm/km and a dispersion slope at1550 nm of less than 0.1 ps/nm²/km. Each core 90 may have the same ordifferent dispersion and dispersion slope at 1550 nm. For example, thedispersion at 1550 nm can be from about 10 ps/nm/km to about 22ps/nm/km, about 10 ps/nm/km to about 22 ps/nm/km, about 10 ps/nm/km toabout 20 ps/nm/km, about 10 ps/nm/km to about 15 ps/nm/km, about 15ps/nm/km to about 22 ps/nm/km, or about 15 ps/nm/km to about 20ps/nm/km. For example, the dispersion at 1550 can be about 10 ps/nm/km,about 15 ps/nm/km, about 16 ps/nm/km, about 17 ps/nm/km, about 17.5ps/nm/km, about 18 ps/nm/km, about 19 ps/nm/km, about 19.5 ps/nm/km,about 19.6 ps/nm/km, about 20 ps/nm/km, about 20.1 ps/nm/km, about 22ps/nm/km, or any value between these values. In one example, thedispersion slope at 1550 nm can be about 0.04 ps/nm^(2/)km to about 0.1ps/nm^(2/)km, about 0.05 ps/nm^(2/)km to about 0.1 ps/nm^(2/)km, about0.055 ps/nm^(2/)km to about 0.1 ps/nm^(2/)km, about 0.06 ps/nm^(2/)km toabout 0.1 ps/nm^(2/)km, about 0.08 ps/nm^(2/)km to about 0.1ps/nm^(2/)km, about 0.04 ps/nm^(2/)km to about 0.08 ps/nm^(2/)km, about0.05 ps/nm^(2/)km to about 0.08 ps/nm^(2/)km, about 0.055 ps/nm^(2/)kmto about 0.08 ps/nm^(2/)km, about 0.06 ps/nm^(2/)km to about 0.08ps/nm^(2/)km, about 0.04 ps/nm^(2/)km to about 0.06 ps/nm^(2/)km, about0.05 ps/nm ^(2/)km to about 0.06 ps/nm^(2/)km, or about 0.055ps/nm^(2/)km to about 0.06 ps/nm^(2/)km. For example, the dispersionslope at 1550 nm can be about 0.04 ps/nm^(2/)km, about 0.05ps/nm^(2/)km, about 0.055 ps/nm^(2/)km, about 0.057 ps/nm^(2/)km, about0.058 ps/nm^(2/)km, about 0.059 ps/nm^(2/)km, about 0.06 ps/nm^(2/)km,about 0.061 ps/nm^(2/)km, about 0.07 ps/nm^(2/)km, about 0.08ps/nm^(2/)km, or any value between these values.

According to one aspect, the bending loss of each core 90 in themulticore optical fibers disclosed herein at 1550 nm, as determined bythe mandrel wrap test using a mandrel having a diameter of 10 mm, may beless than about 3 dB/turn, less than about 2.5 dB/turn, less than about2 dB/turn, less than about 1.5 dB/turn, or less than about 1 dB/turn.For example, the bend loss can be from about 0.5 dB/turn to about 3dB/turn, about 0.5 dB/turn to about 2.5 dB/turn, about 0.5 dB/turn toabout 2 dB/turn, about 0.5 dB/turn to about 1.5 dB/turn, about 0.5dB/turn to about 1 dB/turn, about 1 dB/turn to about 3 dB/turn, about 1dB/turn to about 2.5 dB/turn, about 1 dB/turn to about 2 dB/turn, about1 dB/turn to about 1.5 dB/turn, about 1.5 dB/turn to about 3 dB/turn,about 1.5 dB/turn to about 2.5 dB/turn, about 1.5 dB/turn to about 2dB/turn, about 2 dB/turn to about 3 dB/turn, or about 2 dB/turn to about2.5 dB/turn using a mandrel having a diameter of 10 mm. For example, thebend loss can be about 0.5 dB/turn, about 0.75 dB/turn, about 0.9dB/turn, about 1 dB/turn, about 1.5 dB/turn, about 2 dB/turn, about 2.5dB/turn, about 3 dB/turn, or any value between these values, using amandrel having a diameter of 10 mm.

According to one aspect, the bending loss of each core 90 in themulticore optical fibers disclosed herein at 1550 nm, as determined bythe mandrel wrap test using a mandrel having a diameter of 15 mm, may beless than about 1 dB/turn, less than about 0.75 dB/turn, less than about0.5 dB/turn, or less than about 0.3 dB/turn. For example, the bend losscan be from about 0.1 dB/turn to about 1 dB/turn, about 0.1 dB/turn toabout 0.75 dB/turn, about 0.1 dB/turn to about 0.5 dB/turn, about 0.2dB/turn to about 1 dB/turn, about 0.2 dB/turn to about 0.75 dB/turn,about 0.2 dB/turn to about 0.5 dB/turn, about 0.3 dB/turn to about 1dB/turn, about 0.3 dB/turn to about 0.75 dB/turn, or about 0.3 dB/turnto about 0.5 dB/turn, using a mandrel having a diameter of 15 mm. Forexample, the bend loss can be about 0.2 dB/turn, about 0.23 dB/turn,about 0.25 dB/turn, about 0.3 dB/turn, about 0.5 dB/turn, about 0.6dB/turn, about 0.75 dB/turn, about 1 dB/turn, or any value between thesevalues, using a mandrel having a diameter of 15 mm.

According to one aspect, the bending loss of each core 90 in themulticore optical fibers disclosed herein at 1550 nm, as determined bythe mandrel wrap test using a mandrel having a diameter of 20 mm, may beless than about 3 dB/turn, less than about 2 dB/turn, less than about 1dB/turn, less than about 0.5 dB/turn, less than about 0.3 dB/turn, orless than about 0.2 dB/turn. For example, the bend loss can be fromabout 0.1 dB/turn to about 3 dB/turn, about 0.1 dB/turn to about 2.5dB/turn, about 0.1 dB/turn to about 2 dB/turn, about 0.2 dB/turn toabout 3 dB/turn, about 0.2 dB/turn to about 2.5 dB/turn, about 0.2dB/turn to about 2 dB/turn, about 0.3 dB/turn to about 3 dB/turn, about0.3 dB/turn to about 2.5 dB/turn, or about 0.3 dB/turn to about 2dB/turn, about 0.1 dB/turn to about 1 dB/turn, about 0.1 dB/turn toabout 0.75 dB/turn, about 0.1 dB/turn to about 0.5 dB/turn, 0.5 dB/turnto about 3 dB/turn, about 0.5 dB/turn to about 2.5 dB/turn, about 0.5dB/turn to about 2 dB/turn, 1 dB/turn to about 3 dB/turn, about 1dB/turn to about 2.5 dB/turn, or about 1 dB/turn to about 2 dB/turn,using a mandrel having a diameter of 20 mm. For example, the bend losscan be about 0.2 dB/turn, about 0.23 dB/turn, about 0.25 dB/turn, about0.3 dB/turn, about 0.5 dB/turn, about 0.6 dB/turn, about 0.75 dB/turn,about 0.8 dB/turn, about 0.9 dB/turn, about 1 dB/turn, about 2 dB/turn,about 2.1 dB/turn, about 2.5 dB/turn, about 3 dB/turn, or any valuebetween these values, using a mandrel having a diameter of 20 mm.

According to one aspect, the bending loss of each core 90 in themulticore optical fibers disclosed herein at 1550 nm, as determined bythe mandrel wrap test using a mandrel having a diameter of 30 mm, may beless than about 1 dB/turn, less than about 0.5 dB/turn, less than about0.25 dB/turn, less than about 0.1 dB/turn, less than about 0.05 dB/turn,less than about 0.01 dB/turn, or less than about 0.005 dB/turn. Forexample, the bend loss can be from about 0.01 dB/turn to about 1dB/turn, about 0.01 dB/turn to about 0.5 dB/turn, about 0.01 dB/turn toabout 0.25 dB/turn, about 0.01 dB/turn to about 0.2 dB/turn, about 0.01dB/turn to about 0.1 dB/turn, about 0.01 dB/turn to about 0.005 dB/turn,about 0.05 dB/turn to about 1 dB/turn, about 0.05 dB/turn to about 0.5dB/turn, about 0.05 dB/turn to about 0.25 dB/turn, or about 0.05 dB/turnto about 0.2 dB/turn, about 0.2 dB/turn to about 1 dB/turn, about 0.2dB/turn to about 0.5 dB/turn, or about 0.5 dB/turn to about 1 dB/turn,using a mandrel having a diameter of 30 mm. For example, the bend losscan be about 0.005 dB/turn, about 0.01 dB/turn, about 0.05 dB/turn,about 0.06 dB/turn, about 0.07 dB/turn, about 0.08 dB/turn, about 0.09dB/turn, about 0.1 dB/turn, about 0.12 dB/turn, about 0.13 dB/turn,about 0.15 dB/turn, about 0.2 dB/turn, about 0.23 dB/turn, about 0.24dB/turn, about 0.24 dB/turn, about 0.25 dB/turn, about 0.3 dB/turn,about 0.31 dB/turn, about 0.4 dB/turn, about 0.5 dB/turn, about 0.51dB/turn, about 1 dB/turn, or any value between these values using amandrel having a diameter of 30 mm.

The multicore optical fibers, according to the embodiments of thepresent disclosure, may have the same properties as discussed above withreference to the single-core optical fibers. For example, the glasscores of the multicore optical fibers may have the same trench volume,mode field diameter, effective area, and attenuation as disclosed above.Therefore, the optical fibers comprising the multicore optical fibers ofthe present disclosure may also have reduced profiles while stillmaintaining sufficient microbending and robustness needed for long-haultransmission.

Table 14 below provides examples of multicore optical fibers 310-340 inaccordance with embodiments of the present disclosure. Each multicorefiber 310-340 is formed of 4 cores arranged in a square design, as shownin FIG. 9 . Fibers 310-340 each have an outer glass diameter of 125microns and a coating outer diameter of 242 microns. The radius (r₁) ofeach core of fibers 310-340 is about 17.5 microns, and each core isdoped with chlorine. Furthermore, each core has an off-set trench designand a common cladding disposed outside of the trench (as disclosedherein). The refractive index of the common cladding is −0.245%. Thedistance D1 between a centerline of a first core and a centerline of asecond core in fibers 310-340 is 40 microns. And, distance D3 between acenterline of the cores and an outer radius of the common cladding infibers 310-340 is 34.2 microns. The optical properties of fibers 310-340are shown in Table 14 below, and a relative refractive index profile ofeach core is shown in FIG. 11 .

TABLE 14 Optical Properties of Multicore Optical Fibers 310-340Multicore Optical Fiber 310 Multicore Optical Fiber 320 Length of 45822568 Fiber (m) Core 1 Core 2 Core 3 Core 4 Core 1 Core 2 Core 3 Core 4Cable Cutoff 1198 1199 1181 1188 1154 1156 1157 1165 (nm) Mode Field10.57 10.69 10.50 10.47 10.75 10.55 10.66 10.71 Diameter at 1550 nm (μm)Attenuation at 0.173 0.167 0.191 0.199 0.182 0.185 0.192 0.184 1550 nmloss (dB/km) Polarization 0.112 0.112 0.114 0.105 0.151 0.14 0.14 0.14Mode Dispersion at 1550 nm (ps/{square root over (km))} MulticoreOptical Fiber 330 Multicore Optical Fiber 340 Length of 5429 22186 Fiber(m) Core 1 Core 2 Core 3 Core 4 Core 1 Core 2 Core 3 Core 4 Cable Cutoff1213 1203 1200 1208 1170 1170 1166 1173 (nm) Mode Field 10.68 10.5710.24 12.44 10.35 10.37 10.47 10.55 Diameter at 1550 nm (μm) Attenuationat 0.194 0.165 0.229 0.162 0.203 0.189 0.189 0.188 1550 nm loss (dB/km)Polarization 0.103 0.103 0.096 0.115 0.053 0.061 0.055 0.053 ModeDispersion at 1550 nm (ps/{square root over (km)})

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. An optical fiber comprising: a core regioncomprising silica glass doped with chlorine and/or an alkali metal; acladding region surrounding the core region, the cladding regioncomprising an inner cladding directly adjacent to the core region, anouter cladding surrounding the inner cladding, and a trench regiondisposed between the inner cladding and the outer cladding in a radialdirection, the trench region having a volume of about 30% Δ-micron² orgreater; a primary coating surrounding the cladding region, the primarycoating having an in situ modulus of about 0.5 MPa or less; and asecondary coating surrounding the primary coating, the secondary coatinghaving an in situ modulus of about 1500 MPa or more, wherein a diameterof the secondary coating is about 210 microns or less.
 2. The opticalfiber of claim 1, wherein a thickness of the primary coating is about 25microns or less.
 3. The optical fiber of claim 2, wherein the thicknessof the primary coating is in a range from about 8.0 microns to 20.0microns.
 4. The optical fiber of claim 1, wherein a thickness of thesecondary coating is about 25 microns or less.
 5. The optical fiber ofclaim 4, wherein the thickness of the secondary coating is in a rangefrom about 8.0 microns to 20.0 microns.
 6. The optical fiber of claim 1,wherein a ratio of a thickness of the secondary coating to a thicknessof the primary coating is in a range from about 0.65 to 1.2.
 7. Theoptical fiber of claim 6, wherein the ratio of the thickness of thesecondary coating to the thickness of the primary coating is about 0.80.8. The optical fiber of claim 1, wherein a diameter of the claddingregion is about 130 microns or less.
 9. The optical fiber of claim 8,wherein the diameter of the cladding region is about 110 microns orless.
 10. The optical fiber of claim 1, wherein an in situ glasstransition temperature Tg of the secondary coating is greater than about70° C.
 11. The optical fiber of claim 1, wherein the diameter of thesecondary coating is about 200 microns or less.
 12. The optical fiber ofclaim 11, wherein the diameter of the secondary coating is about 180microns or less.
 13. The optical fiber of claim 1, wherein a diameter ofthe cladding region is about 110 microns or less and the diameter of thesecondary coating is about 200 microns or less.
 14. The optical fiber ofclaim 13, wherein the diameter of the cladding region is about 90microns or less and the diameter of the secondary coating is about 180microns or less.
 15. The optical fiber of claim 1, wherein the coreregion has a peak alkali metal concentration in a range from about 10ppm to about 500 ppm.
 16. The optical fiber of claim 1, wherein thealkali metal is at least one of sodium, potassium, and rubidium.
 17. Theoptical fiber of claim 1, wherein the core region has a chlorineconcentration in a range from about 0.4 wt % to about 2.2 wt %.
 18. Theoptical fiber of claim 1, wherein the volume of the trench region isabout 70% Δ-micron² or less.
 19. The optical fiber of claim 1, wherein apuncture resistance of the optical fiber is about 5 g or more.
 20. Theoptical fiber of claim 19, wherein the puncture resistance of theoptical fiber is about 15 g or more.